[Unicode]  Technical Reports

Unicode Technical Standard #10

Unicode Collation Algorithm



Authors Mark Davis (mark.davis@us.ibm.com), Ken Whistler (ken@unicode.org)


This Version http://www.unicode.org/reports/tr10/tr10-11.html
Previous Version http://www.unicode.org/reports/tr10/tr10-9.html
Latest Version http://www.unicode.org/reports/tr10/
Tracking Version 11


This report provides the specification of the Unicode Collation Algorithm, which provides a specification for how to compare two Unicode strings while remaining conformant to the requirements of The Unicode Standard. The UCA also supplies the Default Unicode Collation Element Table (DUCET) as the data specifying the default collation order for all Unicode characters.


This document has been reviewed by Unicode members and other interested parties, and has been approved by the Unicode Technical Committee as a Unicode Technical Standard. This is a stable document and may be used as reference material or cited as a normative reference by other specifications.

A Unicode Technical Standard (UTS) is an independent specification. Conformance to the Unicode Standard does not imply conformance to any UTS. Each UTS specifies a base version of the Unicode Standard. Conformance to the UTS requires conformance to that version or higher.

Please submit corrigenda and other comments with the online reporting form [Feedback]. Related information that is useful in understanding this document is found in [References]. For the latest version of the Unicode Standard see [Unicode]. For a list of current Unicode Technical Reports see [Reports]. For more information about versions of the Unicode Standard, see [Versions].


1 Introduction
1.1 Multi-Level Comparison
1.2 Canonical Equivalence
1.3 Contextual Sensitivity
1.4 Customization
1.5 Other Applications of Collation
1.6 Interleaved Levels
1.7 Performance
1.8 Common Misperceptions
1.9 Scope
1.9.1 Goals
1.9.2 Non-Goals
2 Conformance
3 Collation Element Table
3.1 Linguistic Features
3.1.1 Multiple Mappings Expansions Contractions Other Multiple Mappings
3.1.2 French Accents
3.1.3 Rearrangement
3.1.4 Default Values
3.1.5 Collation Graphemes
3.2 Default Unicode Collation Element Table
3.2.1 File Format
3.2.2 Variable Weighting
3.3 Well-Formed Collation Element Tables
3.4 Stability
4 Main Algorithm
4.1 Step 1: Normalize
4.2 Step 2: Produce Array
4.3 Step 3: Form Sort Key
4.4 Step 4: Compare
5 Tailoring
5.1 Preprocessing
6 Implementation Notes
6.1 Reducing Sort Key Lengths
6.1.1 Eliminating level separators
6.1.2 L2/L3 in 8 bits
6.1.3 Machine Words
6.1.4 Run-length Compression
6.2 Large Weight Values
6.3 Reducing Table Sizes
6.3.1 Contiguous Weight Ranges
6.3.2 Escape Hatch
6.3.3 Leveraging Unicode tables
6.3.4 Reducing the Repertoire
6.3.5 Memory Table Size
6.4 Avoiding Zero Bytes
6.5 Avoiding Normalization
6.6 Case Comparisons
6.7 Incremental Comparison
6.8 Catching Mismatches
6.9 Tailoring Example: Java
6.10 Flat File Example
6.11.1 Collation Element Format
6.11.2 Sample Code
7 Weight Derivation
7.1 Derived Collation Elements
7.1.1 Illegal code points
7.1.2 Legal code points
7.1.3 Implicit Weights
7.1.4 Trailing Weights
7.2 Canonical Decompositions
7.3 Compatibility Decompositions
7.3.1 Tertiary Weight Table
8 Searching and Matching

1 Introduction

Collation is the general term for the process and function of determining the sorting order of strings of characters. It is a key function in computer systems; whenever a list of strings is presented to users, they are likely to want it in a sorted order so that they can easily and reliably find individual strings. Thus it is widely used in user interfaces. It is also crucial for the operation of databases, not only in sorting records but also in selecting sets of records with fields within given bounds.

However, collation is not uniform; it varies according to language and culture: Germans, French and Swedes sort the same characters differently. It may also vary by specific application: even within the same language, dictionaries may sort differently than phonebooks or book indices. For non-alphabetic scripts such as East Asian ideographs, collation can be either phonetic or based on the appearance of the character. Collation can also be commonly customized or configured according to user preference, such as ignoring punctuation or not, putting uppercase before lowercase (or vice versa), etc. Linguistically correct searching also needs to use the same mechanisms: just as "v" and "w" sort as if they were the same base letter in Swedish, a loose search should pick up words with either one of them.

Thus collation implementations must deal with the often-complex linguistic conventions that communities of people have developed over the centuries for ordering text in their language, and provide for common customizations based on user preferences. And while doing all of this, of course, performance is critical.

The following table shows some examples of cases where sort order differs by language, by usage, or by another customization.

Example Differences



z < ö


ö < z



öf < of


of < öf



A < a


a < A

The conventions that people have developed over the centuries for collating text in their language are often quite complex.  Sorting all Unicode characters in a uniform and consistent manner presents a number of challenges. And for any collation mechanisms to be accepted in the marketplace, algorithms that allow for good performance are crucial.

Languages vary not only regarding which types of sorts to use (and in which order they are to be applied), but also in what constitutes a fundamental element for sorting. For example, Swedish treats ä as an individual letter, sorting it after z in the alphabet; German, however, sorts it either like ae or like other accented forms of a, thus following a. In Slovak, the digraph ch sorts as if it were a separate letter after c. Examples from other languages (and scripts) abound. Languages whose writing systems use upper and lower case typically ignore the differences in case, unless there are no other differences in the text.

For scripts and characters outside the use of a particular language, explicit rules may not exist. For example, Swedish and French have clear and different rules on sorting ä (either after z or as an accented character with a secondary difference from a), but neither defines the ordering of other characters such as Ж, ש, ♫, ∞, ◊, or ⌂.

1.1 Multi-Level Comparison

To address the complexities of language-sensitive sorting, a multilevel comparison algorithm is employed. In comparing two words, for example, the most important feature is the base character: such as the difference between an A and a B. Accent differences are typically ignored, if there are any differences in the base letters. Case differences (uppercase vs. lowercase), are typically ignored, if there are any differences in the base or accents. Punctuation is variable. In some situations a punctuation is treated like a base character. In other situations, it should be ignored if there are any base, accent, or case differences. There may also be a final, tie-breaking level, whereby if there are no other differences at all in the string, the (normalized) code point order is used.

Comparison Levels
Level Description* Examples


Base characters

role < roles < rule



role < rôle < roles



role < Role < rôle



role < role < Role



role < role < role

These examples are in English; the levels may correspond to different features in other languages. Notice that in each example for levels L2 through Ln, the differences on that level (indicated by the underlined characters) are swamped by the stronger-level differences (indicated by the blue text). For example, the L2 example shows that difference between an o and an accented ô is swamped by an L1 difference (the presence or absence of an s). In the last example, the □ represents a format character, which is otherwise completely ignorable.

The core concept is that the primary level (L1) is for the basic sorting of the text, and the non-primary levels (L2..Ln) are for tweaking other linguistic elements in the writing system that are important to users in ordering, but less important than the order of the basic sorting. In practice, not all of these levels may be needed, depending on the user preferences or customizations.

Note: Many people see the Unicode code charts, and expect the characters in their language to be in the "correct" order in the code charts. Because collation varies by language — not just by script — and because multi-level sorting is a requirement, it is not possible to arrange code points for characters so that simple binary string comparison produces the desired collation order for all languages. Separate data tables are required for correct sorting order.

This is worth emphasizing: the sorting weight of characters is not provided by their position in the Unicode code charts.

1.2 Canonical Equivalence

There are a number of cases in Unicode where the same sequence of characters are canonically equivalent; they are essentially the same character but can be represented in different ways. For more information on what this means, see [UAX #15: Unicode Normalization Forms].

For collation, sequences that are canonically equivalent sort the same. In the table below are some examples, where the triple-bar (≡) to means “sorts the same”. For example, the angstrom symbol was encoded for compatibility, and is canonically equivalent to an A-ring. The latter is also equivalent to the decomposed sequence of A plus the combining ring character. The order of certain combining marks in many cases is also irrelevant, so these must be sorted the same, as in the second example. In the third example, we have a composed character that can be decomposed in five different ways, all of which are canonically equivalent.

Canonical Equivalence


≡ Å
≡ A + º

x + . + ^

≡ x + ^ + .

≡ u + ’
≡ ư + .
≡ ụ + ’
≡ u + . + ’
≡ u + ’ + .

1.3 Contextual Sensitivity

Beyond the concept of levels, there are additional complications in certain languages, whereby the comparison is context sensitive: it depends on more than just single characters compared directly against one another.

First are contractions, where two (or more) characters sort as if they were a single base character. In the table below, CH acts like a character after C. Second are expansions, where a single character sorts as if it were two (or more) characters in sorting. In the table below, an Πligature sorts as if it were O + E. Both of these can be combined: that is, two (or more) characters may sort as if they were a different sequence of two (or more) characters. In the example below, for Japanese, a length mark sorts like the vowel of the previous syllable: as an A after KA and as an I after KI.

Context Sensitivity


H < Z, but


OE < Π< OF


カー < カイ, but
キー > キイ

There are some further oddities in the ways that languages work. Normally, all differences in sorting are assessed going from the start to the end of the string. If all of the base characters are the same, the first accent difference determines the final order. In row 1 of the example below, the first accent difference is on the o, so that is what determines the order. In French and a few other languages, however, it is the last accent difference that determines the order, as in row 2.

French Ordering

Normal Accent Ordering

cote < coté < côte < côté

French Accent Ordering

cote < côte < coté < côté

A second issue comes up with Thai and Lao. These scripts are unusual in Unicode in they are not stored in logical order, but in visual order. That means that in the analysis of text, including sorting, a small number of letters have to be reordered.

Thai Ordering

Logical Order Exceptions

เ ก sorts like ก เ

1.4 Customization

In practice, there are additional features of collation that people need control over, which are expressed in user-interfaces and eventually in APIs. Other customizations or user preferences include (but are not limited to) the following:

Note that phonetic sorting of Han characters requires use of either a lookup dictionary of words or, more typically, special construction of programs or databases to maintain an associated phonetic spelling for the words in the text.

1.5 Other Applications of Collation

The same collation behavior has application in other realms than sorting. In particular, searching should behave consistently with sorting. For example, if v and w are treated as identical base letters in Swedish sorting, then they should do so for searching. For searching, the ability to set the maximal strength level is very important.

Selection is the process of using the comparisons between the endpoints of a range, as when using a SELECT command in a database query. It is crucial that the correct range be returned, according to the users expectations. Consider the example of a German businessman making a database selection, such as to sum up revenue in each of of the cities from O... to P... for planning purposes. If behind his back all cities starting with Ö are excluded because the query selection is using a Swedish collation, there is going to be one very unhappy customer.

A sequence of characters considered to be a unit in collation, such as ch in Slovak, represents a tailored grapheme cluster. For applications of this, see UTR #18: Unicode Regular Expression Guidelines [Reports]. For more information on grapheme clusters, see UAX #29: Text Boundaries [Reports].

1.6 Interleaved Levels

Levels may also need to be interleaved. Take, for example, sorting a database according to two fields. The simplest way to sort is field by field, sequentially. This gives us the results in column one in the example below. First all the levels in Field 1 are compared, then all the levels in Field 2. The problem with this approach is that high level differences in the second field are swamped by minute differences in the first field. Thus we get unexpected ordering for the first names.

Merged Fields


Weak 1st


F1L1, F1L2, F1L3,
F2L1, F2L2, F2L3

F2L1, F2L2, F2L3

F1L1, F2L1,
F1L2, F2L2,
F1L3, F2L3

diSilva John
diSilva Fred
di Silva John
di Silva Fred
disílva John
disílva Fred
diSilva John
di Silva John
disílva John
disílva Fred
di Silva Fred
diSilva Fred
diSilva John
di Silva John
disílva John
diSilva Fred
di Silva Fred
disílva Fred

A second way to do this is to ignore all but base-level differences in the sorting of the first field. This gives us the results in column 2. The first names are then all in the right order, but the problem is now that the first field is not correctly ordered except on the base character level.

The correct way to sort is to merge the fields in sorting, as shown in the last column. Using this technique, all differences in the fields are taken into account, and the levels are considered uniformly: accents in all fields are ignored if there are any base character differences in any of the fields; case in all fields are ignored if there are base character differences in any of the fields; and so on.

1.7 Performance

Collation is one of the most performance-critical features in a system. Consider the number of comparison operations that are involved in sorting or searching large databases, for example. Most production implementations will use a number of optimizations to speed up string comparison.

There is a common mechanism for preprocessing strings so that multiple comparisons operations are much faster. With this mechanism, each Collation provides for the generation of a sort key from any given string. The binary comparison of any two sort keys will yield the same result (less, equal, or greater) as the Collation would return for a comparison of the original strings. Thus for a given collation C and any two strings A and B:

A ≤ B according to C if and only if sortkey(C, A) ≤ sortkey(C, B)

Still, simple string comparison is faster for any individual comparison. This is easy to understand, since the generation of a sort key requires processing an entire string, while in most string comparisons differences are found before all the characters are processed. Typically there is a considerable difference in performance, with simple string comparison being about 5 to 10 times faster then generating sort keys and then using a binary comparison.

However, sort keys can be much faster for multiple comparisons. Since binary comparison is blindingly faster than string comparison, whenever there will be more than about 10 comparisons per string — and the system can afford the storage — it is faster to use sort keys.

1.8 Common Misperceptions

There are a number of common misperceptions about collation.

  1. Collation is not aligned with character sets or repertoires of characters. Swedish and German share most of the same characters, for example, but have very different sorting orders.

  2. Collation is not code point (binary) order. The simplest case of this is capital Z vs. lowercase a.

    • As noted above, beginners may complain about Unicode that a particular character is “not in the right place in the code chart”. That is a misunderstanding of the role of the character encoding in collation. While the Unicode Standard does not gratuitously place characters such that the binary ordering is odd, the only way to get the linguistically-correct order is to use a language-sensitive collation, not a binary ordering.

  3. Collation is not a property of strings. Consider a list of cities, with each city correctly tagged with its language. Despite this, a German user will expect to see the cities all sorted according to German order, and not expect to see a word with ö appear after z, simply because the city has a Swedish name. Of crucial importance is that if a German businessman makes a database selection, such as to sum up revenue in each of of the cities from O... to P... for planning purposes, then cities starting with Ö must not be excluded.

  4. Collation order is not preserved under concatenation or substring operations, in general. For example, the fact that x is less than y does not mean that x + z is less than y + z. This is because characters may form contractions across the substring or concatenation boundaries. In summary, the following shows which implications not to expect.

    x < y ↛ xz < yz
    x < y ↛ zx < zy
    xz < yz ↛ x < y
    zx < zy ↛ x < y

  5. Collation order is not preserved when comparing sort keys generated from different Collations. Remember that sort keys are a preprocessing of strings according to a given set of collation features. From different features, you will get different binary sequences. For example, suppose we have two collations, F and G, where F is a French collation (with accents compared from the end), and G is a German phonebook ordering. Then

    • A ≤ B according to F if and only if sortkey(F, A) ≤ sortkey(F, B), and

    • A ≤ B according to G if and only if sortkey(G, A) ≤ sortkey(G, B)

    • But the relation between sortkey(F, A) and sortkey(G, B) says nothing about whether A ≤ B according to F, or whether A ≤ B according to G.

  6. Collation order is not a stable sort; that is a property of a sort algorithm, not a Collation. For more information, see §3.4 Stability.

  7. Collation order is not fixed. Over time, collation order will vary: there may be fixes that are discovered as more information becomes available about languages; there may be new government or industry standards for the language that require changes; and finally, the new characters that are added to Unicode periodically will interleave with the previously-defined ones. Thus collations must be carefully versioned.

1.9 The Unicode Collation Algorithm

The Unicode Collation Algorithm (UCA) provides a specification for how to compare two Unicode strings while remaining conformant to the requirements of The Unicode Standard. The UCA also supplies the Default Unicode Collation Element Table (DUCET), which is data specifying the default collation order for all Unicode characters. This table is designed so that it can be tailored to meet the requirements of different languages and customizations.

Briefly stated, the Unicode Collation Algorithm takes an input Unicode string and a Collation Element Table, containing mapping data for characters. It produces a sort key, which is an array of unsigned 16-bit integers. Two or more sort keys so produced can then be binary-compared to give the correct comparison between the strings for which they were generated.

The Unicode Collation Algorithm assumes multiple-level key weighting, along the lines widely implemented in IBM technology, and as described in the Canadian sorting standard [CanStd] and the proposed International String Ordering standard [SoStd].

By default, the algorithm makes use of three fully-customizable levels. For the Latin script, these levels correspond roughly to:

  1. alphabetic ordering
  2. diacritic ordering
  3. case ordering.

A final level for tie-breaking (semi-stability) may be used for tie-breaking between strings not otherwise distinguished.

This design allows implementations to produce culturally acceptable collation, while putting the least burden on implementations in terms of memory requirements and performance. In particular, Collation Element Tables only require storage of 32 bits of collation data per significant character.

However, implementations of the Unicode Collation Algorithm are not limited to supporting only 3 levels. They are free to support a fully customizable 4th level (or more levels), as long as they can produce the same results as the basic algorithm, given the right Collation Element Tables. For example, an application which uses the algorithm, but which must treat some collection of special characters as ignorable at the first 3 levels and must have those specials collate in non-Unicode order (as, for example to emulate an existing EBCDIC-based collation), may choose to have a fully customizable 4th level. The downside of this choice is that such an application will require more storage, both for the Collation Element Table and in constructed sort keys.

The Collation Element Table may be tailored to produce particular culturally required orderings for different languages or locales. As in the algorithm itself, the tailoring can provide full customization for three (or more) levels.

1.9.1 Goals

The algorithm is designed to satisfy the following goals:

  1. A complete, unambiguous, specified ordering for all characters in Unicode.
  2. A complete resolution of the handling of canonical and compatibility equivalences as relates to the default ordering.
  3. A complete specification of the meaning and assignment of collation levels, including whether a character is ignorable by default in collation.
  4. A complete specification of the rules for using the level weights to determine the default collation order of strings of arbitrary length.
  5. Allowance for override mechanisms (tailoring) for creating language-specific orderings. Tailoring can be provided by any well-defined syntax that takes the default ordering and produces another well-formed ordering.
  6. An algorithm that can be efficiently implemented, both in terms of performance and in terms of memory requirements.

Given the standard ordering and the tailoring for any particular language, any two companies or individuals — with their own proprietary implementations — can take any arbitrary Unicode input and produce exactly the same sorted output. In addition, when given a tailoring specifying French accents this algorithm passes the Canadian and ISO 14651 benchmarks ([CanStd], [SoStd]).

Note: The Default Unicode Collation Element Table does not explicitly list weights for all assigned Unicode characters. However, the algorithm is well defined over all Unicode code points. See §7.1.2 Legal code points.

1.9.2 Non-Goals

The Default Unicode Collation Element Table explicitly does not provide for the following features:

  1. reversibility: from a Collation Element you are not guaranteed that you can recover the original character.
  2. numeric formatting: numbers composed of a string of digits or other numerics will not necessarily sort in numerical order.
  3. API: no particular API is specified or required for the algorithm.
  4. title sorting: for example, removing articles such as a and the during bibliographic sorting is not provided.
  5. Stability of binary sort key values between versions: For more information, see §3.4 Stability.
  6. linguistic applicability: to meet most user expectations, a linguistic tailoring is needed. For more information, see §5 Tailoring.

2 Conformance

There are many different ways to compare strings, and the Unicode Standard does not restrict the ways in which implementations can do this. However, any Unicode-conformant implementation that purports to implement the Unicode Collation Algorithm must do so as described in this document.

Note: A conformance test for the UCA is available in [Test].

The algorithm is a logical specification, designed to be straightforward to describe. Actual implementations of the algorithm are free to change any part of the algorithm so long as any two strings compared by the implementation are ordered the same as they would be by the algorithm. They are also free to use a different format for the data in the Collation Element Table. The sort key is also a logical intermediate object: so long as an implementation produces the same results in comparison of strings, the sort keys can differ in format from what is specified here. (See §6 Implementation Notes.)

The requirements for conformance on implementations of the Unicode Collation Algorithm are as follows:


Given a well-formed Unicode Collation Element Table, a conformant implementation shall replicate the same comparisons of strings as those produced by §4 Main Algorithm.

In particular, a conformant implementation must be able to compare any two canonically equivalent strings as being equal, for all Unicode characters supported by that implementation.

If a conformant implementation compares strings in a legacy character set, it must provide the same results as if those strings had been transcoded to Unicode.


A conformant implementation shall support at least three levels of collation.

A conformant implementation is only required to implement three levels. However, it may implement four (or more) levels if desired.


A conformant implementation that supports backward levels, variable weighting, semi-stability or rearrangement shall do so in accordance with this specification.

A conformant implementation is not required to support these features; however, if it does so, it must interpret them properly. Unless they are functioning in a very restricted domain, it is strongly recommended that implementations support a backwards secondary level, since this is required for French.

C4 A conformant implementation must specify the version number of this Unicode Technical Standard.

The precise values of the collation elements for the characters may change over time as new characters are added to the Unicode Standard. The base Unicode version for this technical standard is the same as the version of this document.

3 Collation Element Table

A Collation Element Table contains a mapping from one (or more) characters to one (or more) collation elements, where a collation element is an ordered list of three 16-bit weights. (All code points not explicitly mentioned in the mapping are given an implicit weight: see §7 Weight Derivation).

Note: Implementations can produce the same result without using 16-bit weights — see §6 Implementation Notes.

The first weight is called the Level 1 weight (or primary weight), the second is called the Level 2 weight (secondary weight), the third is called the Level 3 weight (tertiary weight), the fourth is called the Level 4 weight (quaternary weight), and so on. For a collation element X, these can be abbreviated as X1, X2, X3, X4, etc. Given two collation elements X and Y, we will use the following notation:

Equals Notation
Notation Reading Meaning
X =1 Y X is primary equal to Y X1 = Y1
X =2 Y X is secondary equal to Y X2 = Y2 and X =1 Y
X =3 Y X is tertiary equal to Y X3 = Y3 and X =2 Y
X =4 Y X is quaternary equal to Y X4 = Y4 and X =3 Y

Less Than Notation
Notation Reading Meaning
X <1 Y X is primary less than Y X1 < Y1
X <2 Y X is secondary less than Y X <1 Y or (X =1 Y and X2 < Y2)
X <3 Y X is tertiary less than Y X <2 Y or (X =2 Y and X3 < Y3)
X <4 Y X is quaternary less than Y X <3 Y or (X =3 Y and X4 < Y4)

Other operations are given their customary definitions in terms of these. That is:

Note: Where only plain text ASCII characters are available the following fallback notation can be used:

Notation Fallback
X <n Y X <[n] Y
Xn X[n]
X n Y X <=[n] Y
A B A =[a] B

The collation algorithm results in a similar ordering among characters and strings, so that for two strings A and B we can write A <2 B, meaning that A is less than B and there is a primary or secondary difference between them. If A <2 B but A=1 B, we say that there is only a secondary difference between them. If two strings are equivalent (equal at all levels) according to a given Collation Table, we write A B. If they are bit-for-bit identical, we write A = B.

If a weight is 0000, then that collation element is ignorable at that level: the weight at that level is not taken into account in sorting. A Level N ignorable is a collation element that is ignorable at level N but not at level N+1. Thus:

In addition:

For a given Collation Element Table, MINn is the least weight in any collation element at level n, and MAXn is the maximum weight in any collation element at level n.

The following are sample collation elements that are used in the examples illustrating the algorithm. Unless otherwise noted, all weights are in hexadecimal format.

Sample Table


Collation Element


0300 "`" [0000.0021.0002] COMBINING GRAVE ACCENT
0061 "a" [06D9.0020.0002] LATIN SMALL LETTER A
0062 "b" [06EE.0020.0002] LATIN SMALL LETTER B
0063 "c" [0706.0020.0002] LATIN SMALL LETTER C
0043 "C" [0706.0020.0008] LATIN CAPITAL LETTER C
0064 "d" [0712.0020.0002] LATIN SMALL LETTER D

Note: Weights in all examples are illustrative, and may not match what is in the latest Default Unicode Collation Element Table.

3.1 Linguistic Features

The following section describes the implications of the features discussed in §1 Introduction.

3.1.1 Multiple Mappings

The mapping from characters to collation elements may not be a simple mapping from one character to one collation element: in general, it may map from one to many, from many to one, or from many to many. For example: Expansions

The Latin letter æ is treated as an independent letter by default. Collations such as English, which may require treating it as equivalent to an <a e> sequence, can tailor the letter to map to a sequence of more than one collation elements, such as in the following example:



Collation Element


00E6 [06D9.0020.0002], [073A.0020.0002] LATIN SMALL LETTER AE; "æ"

In this example, the collation element [06D9.0020.0002] gives the weight values for a, and the collation element [073A.0020.0002] gives the weight values for e. Contractions

Similarly, where ch is treated as a single letter as in traditional Spanish, it is represented as a mapping from two characters to a single Collation Element, such as in the following example


Collation Element


[0707.0020.0002] LATIN SMALL LETTER C,

In this example, the collation element [0707.0020.0002] has a primary value one greater than the primary value for the letter c by itself, so that the sequence ch will collate after c and before d. The above example shows the result of a tailoring of collation elements to weight sequences of letters as a single unit.

Any character (such as soft hyphen) that is not completely ignorable between two characters of a contraction will cause them to sort as separate characters.Thus a soft hyphen can be used to separate and cause distinct weighting of sequences such as Slovak ch or Danish aa that would normally weight as units. Other Multiple Mappings

Certain characters may both expand and contract: see Section 5.17 Sorting and Searching.

3.1.2 French Accents

In some languages (notably French), accents are sorted from the back of the string to the front of the string. This behavior is not marked in the Default Unicode Collation Element Table, but may occur in tailored tables. In such a case, the collation elements for the accents and their base characters are marked as being backwards at Level 2.

3.1.3 Rearrangement

Certain characters are not coded in logical order, such as the Thai vowels เ through ไ and the Lao vowels ເ through ໄ (this list is indicated by the Logical_Order_Exception property). For collation, they are rearranged by swapping with the following character before further processing, since logically they belong afterwards. For example, here is a string processed by rearrangement:

input string: 0E01 0E40 0E02 0E03
normalized string: 0E01 0E02 0E40 0E03

3.1.4 Default Values

Both in the Default Unicode Collation Element Table and in typical tailorings, most unaccented letters differ in the primary weights, but have secondary weights (such as a1) equal to MIN2. The primary ignorables will have secondary weights greater than MIN2. Characters that are compatibility or case variants will have equal primary and secondary weights (e.g. a1 = A1 and a2 = A2), but have different tertiary weights (e.g. a3 < A3). The unmarked characters will have tertiary weights (such as a3) equal to MIN3.

However, a well-formed Unicode Collation Element Table does not guarantee that the meaning of a secondary or tertiary weight is uniform across tables. For example, a capital A and katakana ta could both have a tertiary weight of 3.

3.1.5 Collation Graphemes

A collation ordering determines a collation grapheme cluster (also known as a collation grapheme or collation character), which is a sequence of characters that is treated as a primary unit by the ordering. For example, ch is a collation grapheme for a traditional Spanish ordering. These are generally contractions, but may include additional ignorable characters. To determine the boundaries for a collation grapheme starting at a given position, use the following process.

  1. Set oldPosition to be equal to position.
  2. If position is at the end of the string, return it.
  3. Fetch the next collation element(s) mapped to by the character(s) at position.
  4. If the collation element(s) contain a non-ignorable and position is not equal to oldPosition, return position.
  5. Otherwise set position to be the end of the characters mapped.
  6. Loop back to step 2.

For information on the use of collation graphemes, see UTR #18: Unicode Regular Expression Guidelines.

3.2 Default Unicode Collation Element Table

The Default Unicode Collation Element Table is provided in [AllKeys]. This table provides a mapping from characters to collation elements for all the explicitly weighted characters. The mapping lists characters in the order that they would be weighted. Any code points that are not explicitly mentioned in this table are given a derived collation element, as described in §7 Weight Derivation. There are three types of mappings:

This table is constructed to be consistent with the Unicode Canonical Equivalence algorithm, and to respect the Unicode character properties. It is not, however, merely algorithmically derivable from those data, since the assignment of levels does take into account characteristics of particular scripts. For example, in general the combining marks are Level 1 ignorables; however, the Indic combining vowels are given non-zero Level 1 weights, since they are as significant in sorting as the consonants.

Any character may have variant forms or applied accents which affect collation. Thus, for FULL STOP there are three compatibility variants, a fullwidth form, a compatibility form, and a small form. These get different tertiary weights, accordingly. For more information on how the table was constructed, see §7 Weight Derivation.

The following table shows the layout of the collation elements in the Default Unicode Collation Element Table, ordered by primary weight:

DUCET Layout
Values Range Types of Characters
X1, X2, X3 = 0 tertiary ignorables - Control Codes
- Format Characters
- Hebrew Points
- Tibetan Signs
X1, X2 = 0;
X3 ≠ 0
secondary ignorables None in DUCET; could be in tailorings
X1 = 0;
X2, X3 ≠ 0
primary ignorable - Most non-spacing marks
X1, X2, X3 ≠ 0 variable - Whitespace,
- Punctuation,
- Symbols
regular - Small number of exceptional symbols (e.g. U+02D0 (ː) triangular colon)
- Numbers
- Latin
- Greek
implicit - CJK & CJK compatibility (those not decomposed)
- CJK Extension A & B
- Unassigned and others given implicit weights
trailing None in DUCET; could be in tailorings

For most languages, some degree of tailoring is required to match user expectations. For more information, see §5 Tailoring.

3.2.1 File Format

Each of the files consists of a version line followed by an optional variable-weight line, optional rearrangement lines, optional backwards lines, and a series of entries, all separated by newlines. A '#' and any following characters on a line are comments. Whitespace between literals is ignored. The following is an extended BNF description of the format, where "x+" indicates one or more x's, "x*" indicates zero or more x's, "x?" indicates zero or one x, and <char> is a hexadecimal Unicode code value.

<collationElementTable> := <version> 

The version line is of the form:

@<version> := <major>.<minor>.<variant> <eol>

The variable-weight line has three possible values that may change the weights of collation elements in processing (see §3.2.2 Variable Collation Elements). The default is shifted.

<variable>       := '@variable ' <variableChoice> <eol>
<variableChoice> := 'blanked' | 'non-ignorable' | 'shifted'

A backwards line lists a level that is to be processed in reverse order. A forwards line does the reverse. The default is for lines to be forwards.

<backwards> := ('@backwards ' | '@forwards ') <levelNumber> <eol>

Each entry is a mapping from character(s) to collation element(s), and is of the following form:

<entry>       := <charList> ';' <collElement>+ <eol>
<collElement> := "[" <alt> <char> "." <char> "." <char> ("." <char>)* "]"
<alt>         := "*" | "."

In the Default Unicode Collation Element Table, the comment may contain informative tags.

Here are some selected entries taken from a particular version of the data file. (It may not match the actual values in the current data file.)

0020 ; [*0209.0020.0002.0020] % SPACE
02DA ; [*0209.002B.0002.02DA] % RING ABOVE; COMPATSEQ
0041 ; [.06D9.0020.0008.0041] % LATIN CAPITAL LETTER A
3373 ; [.06D9.0020.0017.0041] [.08C0.0020.0017.0055] % SQUARE AU; COMPATSEQ
212B ; [.06D9.002B.0008.212B] % ANGSTROM SIGN; CANONSEQ
0042 ; [.06EE.0020.0008.0042] % LATIN CAPITAL LETTER B
0043 ; [.0706.0020.0008.0043] % LATIN CAPITAL LETTER C
0106 ; [.0706.0022.0008.0106] % LATIN CAPITAL LETTER C WITH ACUTE; CANONSEQ
0044 ; [.0712.0020.0008.0044] % LATIN CAPITAL LETTER D

The entries in each file are ordered by collation element, not by character, using a SHIFED comparison. This makes it easy to see the order in which characters would be collated. Although this document describes collation elements as three levels, the file contains a fourth level (as in [.0712.0020.0008.0044]) which is computable. For more information, see §3.4 Stability.

Implementations can also add more customizable levels, as discussed above under conformance. For example, an implementation might want to be capable not only of handling the standard Unicode Collation, but also capable of emulating an EBCDIC multi-level ordering (having a fourth-level EBCDIC binary order). 

3.2.2 Variable Weighting

Collation elements that are marked with an asterisk in a Unicode Collation Element Table are known as variable collation elements.


Collation Element


0020 " " [*0209.0020.0002] SPACE

Based on the setting of the variable weighting tag, collation elements can be either treated as ignorables or not. When they are treated as ignorables, then any sequence of ignorable characters that immediately follows the variable collation element are also affected.

There are four possible options for variable weighted characters, with the default being Shifted:

Note: The shifted option provides for improved orderings when the variable collation elements are ignorable, while still using only requiring three fields to be stored in memory for each collation element. It does result in somewhat longer sort keys, although they can be compressed (see §6.1 Reducing Sort Key Lengths and §6.3 Reducing Table Sizes).

The following gives an example of the differences between orderings using the different options for variable collation elements. In this example, sample strings differ by the third character: a letter, space, '-' hyphen-minus (002D), or '-' hyphen (2010); followed by an uppercase/lowercase distinction. In the first column below, the words with hyphen-minus and hyphen are separated by deluge, since an l comes between them in Unicode code order. In the second column, they are grouped together but before all letters in the third position. This is because they are no longer ignorable, and have primary values that differ from the letters. In the third column, the hyphen-minus and hyphen are grouped together, and their differences are less significant than between the deluge. In this case, it is because they are ignorable, but their fourth level differences are according to the original primary order, which is more intuitive than Unicode order.





de luge

de Luge


de luge
de Luge


de luge

de Luge


de luge

de Luge


Primaries for variable collation elements are not interleaved with other primary weights. This allows for more compact storage of memory tables. Rather than using a bit per collation element to determine whether the collation element is variable, the implementation only needs to store the maximum primary value for all the variable elements. All collation elements with primary weights from 1 to that maximum are variables; all other collation elements are not.

3.3 Well-Formed Collation Element Tables

A well-formed Collation Element Table meets the following conditions:

  1. Except in special cases detailed in §6.2 Large Weight Values, no collation element can have a zero weight at Level N and a non-zero weight at Level N-1.
    • For example, the secondary can only be ignorable if the primary is.
    • The reason for this will be explained under Step 4 of the main algorithm.
  2. All Level N weights in Level N-1 ignorables must be strictly less than all weights in Level N-2 ignorables.
    • For example, secondaries in non-ignorables must be strictly less than those in primary ignorables:
      • Given collation elements [C, D, E] and [0, A, B], where C ≠ 0 and A ≠ 0
      • Then D must be less than A.
  3. No variable collation element has an ignorable primary.
  4. For all variable collation elements U, V, if there is a collation element W such that U1 W1 and W1 V1, then W is also variable.
    • This provision prevents interleaving, mentioned above.

3.4 Stability

One very common confusion in terms of collation centers around the notion of stability in sorting.

A stable sort is one where two records with a field that compares as equal will retain their order if sorted according to that field. This is a property of the sorting algorithm, not the comparison mechanism. For example, a bubble sort is stable, while a quick sort is not. This is a useful property, but cannot be accomplished by modifications to the comparison mechanism or tailorings.

A semi-stable collation is different. It is a collation where strings that are not canonical equivalents will not be judged to be equal. This is a property of comparison, not the sorting algorithm. In general this is not a particularly useful property; its implementation also typically requires extra processing in string comparison or an extra level in sort keys, thus may degrade performance to little purpose. However, if a semi-stable collation is required, the specified mechanism is to append the NFD form of the original string after the sort key, in step 3.10 below.

The fourth-level weights in the Default Collation Element Table can be used to provide an approximation of a semi-stable collation.

Neither one of the above refers to the stability of the Default Collation Element Table itself. For any particular version of the UCA, the contents of that table will remain unchanged. The contents may, however, change between successive versions of the UCA, as new characters are added, or as more information is obtained about existing characters.

Implementers should be aware that using different versions of the UCA, as well as different versions of the Unicode Standard, could result in different collation results of their data. There are numerous ways collation data could vary across versions, for example:

  1. Code points that were unassigned in a previous version of the Unicode Standard are now assigned in the current version, and as such, will have a sorting semantic appropriate to the repertoire to which they belong. For example, the code points U+103D0..U+103DF were undefined in Unicode 3.1. Since they were assigned characters in Unicode 3.2, their sorting semantics and respective sorting weights will change.
  2. Certain semantics of the Unicode standard could change between versions, such that code points are treated in a manner different than previous versions of the standard (for example, normalization errata).
  3. More information is gathered about a particular script, and in order to provide a more linguistically accurate sort, the weight of a code point may need to be adjusted.

Any of these reasons could necessitate a change between versions with regards to sort weights for code points, and as such, it is important that the implementers specify the version of the UCA as well as the version of the Unicode standard under which their data is sorted.

4 Main Algorithm

The main algorithm has four steps. First is to normalize each input string, second is to produce an array of collation elements for each string, and third is to produce a sort key for each string from the collation elements. Two sort keys can then be compared with a binary comparison; the result is the ordering for the original strings.

4.1 Normalize each input string

Step 1. Produce a normalized form of each input string, applying S1.1, S1.2, and S1.3.

S1.1 Use the Unicode canonical algorithm to decompose characters according to the canonical mappings. That is, put the string into Normalization Form D (see UTR #15: Unicode Normalization Forms).

S1.2 If any character has the Logical_Order_Exception property (see §3.1.3 Rearrangement), swap it and the succeeding character (if there is one). In practice, rearranging characters should never appear adjacent to one another. If for some reason they do, then successive pairs in the sequence will be swapped.

S1.3 If any character is marked as ignorable at all levels, remove it from the string.

For example, if a Control-A (U+0001) has a collation element [0000.0000.0000], then it will be removed.


input string:


normalized string:



4.2 Produce an array of collation elements for each string

Step 2. The collation element array is built by sequencing through the normalized form as follows:

Note: A combining mark in a string is called blocked if there is another combining mark of the same canonical combining class or zero between it and the last character of canonical combining class 0.

S2.1 Find the longest initial substring S at each point that has a match in the table.

S2.1.1 If there are any combining marks following S, process each combining mark C.

S2.1.2 If C is not blocked, find if S + C has a match in the table.

S2.1.3 If there is a match, replace S by S + C, and remove C.

S2.2 Fetch the corresponding collation element(s) from the table if there is a match. If there is no match, synthesize a weight as described in §7.1 Derived Collation Elements

S2.3 Process collation elements according to the variable-weight setting, as described in §3.2.2 Variable Weighting.

S2.4 Append the collation element(s) to the collation element array.

S2.5 Proceed to the next point in the string (past S).

S2.6 Loop until the end of the string is reached.

Conformant implementations may skip steps 2.1.1 through 2.1.3 if their repertoire of supported character sequences does not require this level of processing.

Note: The reason for considering the extra combining marks C is that otherwise irrelevant characters could interfere with matches in the table. For example, suppose that the contraction <a, combining_ring> (= å) is ordered after z. If a string consists of the three characters <a, combining_ring, combining_cedilla>, then the normalized form is <a, combining_cedilla, combining_ring>, which separates the a from the combining_ring. If we didn't have the step of considering the extra combining marks, this string would compare incorrectly as after a and not after z.

If the desired ordering treats <a, combining_cedilla> as a contraction which should take precedence over <a, combining_ring>, then an additional mapping for the combination <a, combining_ring, combining_cedilla> can be introduced to produce this effect.

Note: For conformance to Unicode canonical equivalence, only unblocked combining marks are matched. For example, <a, combining_macron, combining_ring> would compare as after a-macron, and not after z. As in the previous note, additional mappings can be added to customize behavior.

normalized string: ca´b
collation element array: [0706.0020.0002], [06D9.0020.0002], [0000.0021.0002], [06EE.0020.0002]

4.3 Form a sort key for each string

Step 3. The sort key is formed by successively appending weights from the collation element array. The weights are appended from each level in turn, from 1 to 3. (Backwards weights are inserted in reverse order.)

An implementation may allow the maximum level to be set to a smaller level than the available levels in the collation element array. For example, if the maximum level is set to 2, then level 3 and higher weights are not appended to the sort key. Thus any differences at levels 3 and higher will be ignored, leveling any such differences in string comparison.

Here is a more detailed statement of the algorithm:

S3.1 For each weight level L in the collation element array from 1 to the maximum level,

S3.2 If L is not 1, append a level separator*

S3.3 If the collation element table is forwards at level L,

S3.4 For each collation element CE in the array

S3.5 Append CEL to the sort key if CEL is non-zero.

S3.6 Else the collation table is backwards at level L, so

S3.7 Form a list of all the non-zero CEL values.

S3.8 Reverse that list

S3.9 Append the CEL values from that list to the sort key.

* The level separator is zero (0000), which is guaranteed to be lower than any weight in the resulting sort key. This guarantees that when two strings of unequal length are compared, where the shorter string is a prefix of the longer string, the longer string is always sorted after the shorter (in the absence of special features like contractions). For example:

"abc" < "abcX" where "X" can be any character(s)

S3.10 If a semi-stable sort is required, then after all the level weights have been added, append a copy of the NFD version of the original string.

collation element array: [0706.0020.0002], [06D9.0020.0002], [0000.0021.0002], [06EE.0020.0002]
sort key: 0706 06D9 06EE 0000 0020 0020 0021 0020 0000 0002 0002 0002 0002

4.4 Compare the sort keys

Step 4. Compare the sort keys for each of the input strings, using a binary comparison. This means that:



Sort Key

cab 0706 06D9 06EE 0000 0020 0020 0020 0000 0002 0002 0002
Cab 0706 06D9 06EE 0000 0020 0020 0020 0000 0008 0002 0002
cáb 0706 06D9 06EE 0000 0020 0020 0021 0020 0000 0002 0002 0002 0002
dab 0712 06D9 06EE 0000 0020 0020 0020 0000 0002 0002 0002

In this example, "cab" <3 "Cab" <2 "cáb" <1 "dab". The differences that produce the ordering are shown by the bold underlined items:

Note: At this point we can explain the reason for disallowing ill-formed weights. If ill-formed weights were allowed, the ordering of elements can be incorrectly reflected in the sort key. For example, suppose the secondary weights of the Latin characters were zero (ignorable) and that (as normal) the primary weights of case-variants are equal: that is, a1 = A1. Then the following incorrect keys would be generated:

  1. "áe" = <a, acute, e> => [a1 e1 0000 acute2 0000 a3 acute3 e3...]
  2. "Aé" = <A, e, acute> => [a1 e1 0000 acute2 0000 A3 acute3 e3...]

Since the secondary weights for a, A, and e are lost in forming the sort key, the relative order of the acute is also lost, resulting in an incorrect ordering based solely on the case of A vs a. With well-formed weights, this does not happen, and you get the following correct ordering:

  1. "Aé" = <A, e, acute> => [a1 e1 0000 a2 e2 acute2 0000 a3 acute3 e3...]
  2. "áe" = <a, acute, e> => [a1 e1 0000 a2 acute2 e2 0000 A3 acute3 e3...]

However, there are circumstances--typically in expansions--where higher-level weights in collation elements can be zeroed (resulting in ill-formed collation elements) without consequence (see §6.2 Large Weight Values). Implementations are free to do this as long as they produce the same result as with well-formed tables.

5 Tailoring

Tailoring is any well-defined syntax that takes the Default Unicode Collation Element Table and produces another well-formed Unicode Collation Element Table. This syntax can provide linguistically-accurate collation, if desired. Such syntax will usually allow for the following capabilities:

  1. Reordering any character (or contraction) with respect to others in the standard ordering. Such a reordering can represent a Level 1 difference, Level 2 difference, Level 3 difference, or identity (in levels 1 to 3). Since such reordering includes sequences, arbitrary multiple mappings can be specified.

  2. Setting the secondary level to be backwards (French) or forwards (normal).

  3. Set variable weighting options.

  4. Customizing the exact list of variable collation elements.

For examples of tailoring syntax, see §6.9 Tailoring Example: Java.

5.1 Preprocessing

In addition to tailoring, some implementation may choose to preprocess the text for special purposes. Once such preprocessing is done, the standard algorithm can be applied.

Examples include:

Such preprocessing is outside of the scope of this document.

6 Implementation Notes

As noted above for efficiency, implementations may vary from this logical algorithm so long as they produce the same result. The following items discuss various techniques that can be used for reducing sort key length, reducing table sizes, customizing for additional environments, searching, and other topics.

6.1 Reducing Sort Key Lengths

The following discuss methods of reducing sort key lengths. If these methods are applied to all of the sort keys produced by an implementation, they can result in significantly shorter and more efficient sort keys while retaining the same ordering.

6.1.1 Eliminating level separators

Level separators are not needed between two levels in the sort key, if the weights are properly chosen. For example, if all L3 weights are less than all L2 weights, then no level separator is needed between them. If there is a fourth level, then the separator before it needs to be retained.

For example, here is a sort key with these level separators removed.


Sort Key

càb (0) 0706 06D9 06EE 0000 0020 0020 0021 0020 0000 0002 0002 0002 0002
càb (1) 0706 06D9 06EE 0020 0020 0021 0020 0002 0002 0002 0002

While this technique is relatively easy to implement, it can interfere with other compression methods.

6.1.2 L2/L3 in 8 bits

The L2 and L3 weights commonly are small values. Where that condition occurs for all possible values, they can then be represented as single 8-bit quantities.

Here is the above example with both these changes (and grouping by bytes). Note that the separator has to remain after the primary weight when combining these techniques. If any separators are retained (such as before the fourth level), they need to have the same width as the previous level.


Sort Key

càb (0) 07 06 06 D9 06 EE 00 00 00 20 00 20 00 21 00 20 00 00 00 02 00 02 00 02 00 02
càb (1,2) 07 06 06 D9 06 EE 00 00 20 20 21 20 02 02 02 02

6.1.3 Machine Words

The sort key can be represented as an array of different quantities depending on the machine architecture. For example, comparisons as arrays of 32-bit quantities may be much faster on some machines. If this is done, the original is to be padded with trailing (not leading) zeros as necessary.


Sort Key

càb (1,2) 07 06 06 D9 06 EE 00 00 20 20 21 20 02 02 02 02
càb (1,2,3) 070606D9 06EE0000 20202120 02020202

6.1.4 Run-length Compression

Generally sort keys don't differ much in the secondary or tertiary weights, so you tend to end up with keys with a lot of repetition. This also occurs with quaternary weights generated with the shifted parameter. By the structure of the collation element tables, there are also many weights that are never assigned at a given level in the sort key. You can take advantage of these regularities in these sequences to compact the length — while retaining the same sort sequence — by using the following technique. (There are other techniques that can also be used.)

This is a logical statement of the process: the actual implementation can be much faster and performed as the sort key is being generated.


Original Weights

Compressed Weights

77 01
77 02
77 77 01
77 77 02
77 77 77 01
77 77 77 02
77 77 77 FE
77 77 77 FF
77 77 FE
77 77 FF
77 FE
77 FF
03 01
03 02
04 01
04 02
05 01
05 02

The result of this process are keys that are never greater than the original, are generally much shorter, and result in the same comparisons.

6.2 Large Weight Values

If a collation sequence requires more than 65,535 weight values (or 65,024 values where zero bytes are avoided), this can still be accommodated by using multiple collation elements for a single character. For example, suppose that 50,000 UTF-16 supplementary characters are assigned in a particular implementation, and that these are to be sorted after X. Simply assign them all dual collation elements of the form

[(X1+1).0000.0000], [yyyy.zzzz.wwww]

They will then sort properly with respect to each other and to the rest of the characters. (The first collation element is one of the instances where ill-formed collation elements are allowed. Since the second collation element is well-formed and the first element will only occur in combination, ordering is preserved.)

6.3 Reducing Table Sizes

The data tables required for full Unicode sorting can be quite sizable. This section discusses ways to significantly reduce the table size in memory. These have very important implications for implementations.

6.3.1 Contiguous Weight Ranges

The Default Unicode Collation Element Table has secondary weights that are greater than 00FF. This is the result of the derivation described in §7 Weight Derivation. However, these values can be compacted to a range of values that don't exceed 00FF. Whenever collation elements have different primary weights, the ordering of their secondary weights is immaterial. Thus all of the secondaries that share a single primary can be renumbered to a contiguous range without affecting the resulting order. Composite characters still need to be handled correctly if normalization is avoided as discussed in §7 Weight Derivation.

For example, for the primary value 0820 (for the letter O), there are 31 distinct secondary values ranging from 0020 to 012D. These can be renumbered to the contiguous range from 0020 to 003F, which is less than 00FF.

6.3.2 Escape Hatch

Although the secondary and tertiary weights for the Default Unicode Collation Element Table can both fit within one byte, of course, any particular tailored table could conceivably end up with secondary or tertiary weights that exceed what can be contained in a single byte. However, the same technique used for large weight values can also be used for implementations that do not want to handle more than 00FF values for a particular weight.

For example, the Java collation implementation only stores 8-bit quantities in level 2 and level 3. However, characters can be given L2 or L3 weights with greater values by using a series of two collation elements. For example, with characters requiring 2000 weights at L2, then 248 characters can be given single keys, while 1792 are given 2 collation keys of the form [yyyy.00zz.00ww] [0000.00nn.0000]. (The 248 can be chosen to be the higher frequency characters!)

6.3.3 Leveraging Unicode Tables

Since all canonically decomposable characters are decomposed in Step 1.1, no collation elements need to be supplied for them. This includes a very large number of characters, not only a large number of Latin and Greek characters, but also the very large number of Hangul Syllables.

Since most compatibility decomposable characters in the default table can be algorithmically generated from the decomposition, no collation elements need to be stored for those decomposable characters: the collation elements can be generated on the fly with only a few exceptions entered in the table. The collation elements for the Han characters (unless tailored) are algorithmically derived; no collation elements need to be stored for them either. For more information, see §7 Weight Derivation.

This means that only a fraction of the total number of Unicode characters needs to have an explicit collation element associated with them. This can cut down the memory storage considerably.

6.3.4 Reducing the Repertoire

If characters are not fully supported by an implementation, then their code points can be treated as if they were unassigned. This allows them to be algorithmically constructed from code point values instead of including them in a table. This can significantly reduce the size of the required tables. See §7.1 Derived Collation Elements for more information.

6.3.5 Memory Table Size

Applying the above techniques, an implementation can thus safely pack all of the data for a collation element into a single 32-bit quantity: 16 for the primary, 8 for the secondary and 8 for the tertiary. Then applying techniques such as the Two-Stage table approach described in Section 5.7 of The Unicode Standard, Version 2.0, the mapping table from characters to collation elements can both fast and small. For an example of how this can be done, see §6.11 Flat File Example.

6.4 Avoiding Zero Bytes

If the resulting sort key is to be a C-string, then zero bytes must be avoided. This can be done by:

6.5 Avoiding Normalization

Implementations that do not handle separate combining marks can map decomposable characters (such as "à") to single collation elements with different Level 2 weights for the different accents. For more information, see §7 Weight Derivation. However, this does required including the mappings for these characters in the collation table, which will increase the size substantially unless the collation elements for the Hangul Syllables are computed algorithmically.

6.6 Case Comparisons

In some languages, it is common to sort lowercase before uppercase; in other languages this is reversed. Often this is more dependent on the individual concerned, and is not standard across a single language. It is strongly recommended that implementations provide parameterization that allow uppercase to be sorted before lowercase, and provide information as to the standard (if any) for particular countries. This can easily be done to the Default Unicode Collation Element Table before tailoring by remapping the L3 weights (see §7 Weight Derivation). It can be done after tailoring by finding the case pairs and swapping the collation elements.

6.7 Incremental Comparison

Implementations do not actually have to produce full sort keys. Collation elements can be incrementally generated as needed from two strings, and compared with an algorithm that produces the same results as sort keys would have. The choice of which algorithm to use depends on the number of comparisons between the same strings.

However, it is very tricky to produce an incremental comparison that produces correct results. For example, some implementations have not even been transitive! Be sure to test any code for incremental comparison thoroughly.

6.8 Catching Mismatches

Sort keys from two different tailored collations cannot be compared, since the weights may end up being rearranged arbitrarily. To catch this case, implementations can produce a hash value from the collation data, and prepend it to the sort key. Except in extremely rare circumstances, this will distinguish the sort keys. The implementation then has the opportunity to signal an error.

6.9 Tailoring Example: Java

Java 2 implements a number of the tailoring features described in this document. The following summarizes these features (for more information, see Collator on [JavaCollator]).

1. Java doesn't use a default table in the Unicode Collation Element format: instead it always uses a tailoring syntax. Here is a description of the entries:

Java Syntax Description
 & y < x Make x primary-greater than y
 & y ; x Make x secondary-greater than y
 & y , x Make x tertiary-greater than y
 & y = x Make x equal to y

Either x or y can be more than one character, to handle contractions and expansions. NULL is completely ignorable, so by using the above operations, various levels of ignorable characters can be specified.

2. Entries can be abbreviated in a number of ways:

These can be done successively, so the following are equivalent in ordering.


Unicode Collation Element Table

 a, A ; à, À < b, B
0061 ; [.0001.0001.0001] % a
0040 ; [.0001.0001.0002] % A
00E0 ; [.0001.0002.0001] % à
00C0 ; [.0001.0002.0002] % à
0042 ; [.0002.0001.0001] % b
0062 ; [.0002.0001.0002] % B

For a discussion of more powerful tailoring features, see [ICUCollator]. For details on a common XML format for tailorings, see [LDML].

6.10 Flat File Example

The following is a sample flat-file binary layout and sample code for collation data. It is included only for illustration. The table is used to generate collation elements from characters, either going forwards or backwards, and detect the start of a contraction. The backwards generation is for searching backwards or Boyer-Moore-style searching; the contraction detection is for random access.

In the file representation, ints are 32 bit values, shorts are 16, bytes are 8 bits. Negatives (not that we have any) are two's-complement. For alignment, the ends of all arrays are padded out to multiples of 32 bits. The signature determines endianness. The locale uses an ASCII representation for the Java locale: a 2 byte ISO language code, optionally followed by '_' and 2 byte ISO country code, followed optionally by a series of variant tags separated by '_'; any unused bytes are zero.

Data Comment
int signature; Constant 0x636F6C74, used also for big-endian detection
int tableVersion; Version of the table format
int dataVersion; Version of the table data
byte[32] locale; Target locale (if any)
int flags; Bit01 = 1 if French secondary
Others are reserved
int limitVariable; Every ce below this value that has a non-zero primary is variable. Since variables are not interleaved, this does not need to be stored on a per-character basis.
int maxCharsPerCE; Maximum number of characters that are part of a contraction
int maxCEsPerChar; Maximum number of collation elements that are generated by an expansion
int indexOffset; Offset to index table
int collationElementsOffset; Offset to main data table
int expansionsOffset; Offset to expansion table
int contractionMatchOffset; Offset to contraction match table
int contractionResultOffset; Offset to contraction values table
int nonInitialsOffset; Offset to non-initials table. These are used for random access.
int[10] reserved; Reserved
int indexLength; Length of following table
int[] index; Index for high-byte (trie) table. Contains offsets into Collation Elements. Data is accessed by:
ce = collationElements[index[char>>8]+char&0xFF]
int collationElementsLength; Length of following table
int[] collationElements; Each element is either a real collation element, an expansionsOffset, or an contractionsOffset. See below for more information.
int expansionsLength; Length of following table
int[] expansions; The expansionOffsets in the collationElements table point into sublists in this table. Each list is terminated by FFFFFFFF.
int contractionMatchesLength; Length of following table
short[] contractionMatches; The contractionOffsets in the collationElements table point into sublists in this table. Each sublist is of the following format:
short backwardsOffset; In case we are going backwards, offset to true contractions table.
short length; Number of chars in list to search
short[] charsToMatch; characters in sorted order.
int contractionCEsLength; Length of following table
int[] contractionCEs; List of CEs. Each corresponds to a position in the contractionChars table. The one corresponding to the length in a sublist is the bail-out; what to do if a match is not found.
int nonInitialsLength; Length of following table
short[] nonInitials; List of characters (in sorted order) that can be non-initials in contractions. That is, if "ch" is a contraction, then "h" is in this list. If "abcd" is a contraction, then "b", "c", and "d" are in the list.

6.11.1 Collation Element Format

An alternative structure would be to have the offsets be not indexes into the arrays, but byte offsets from the start of the table. That would limit the size of the table, but use fewer machine instructions.

6.11.2 Sample Code

The following is a pseudo code using this table for the required operations. Although using Java syntax in general, the code example uses arrays so as to be more familiar to users of C and C++. The code is presented for illustration only; it is not a complete statement of the algorithm. 

char[] input;   // input buffer (i)
int inputPos;   // position in input buffer (io)
int[] output;   // output buffer (o)
int outputPos;  // position in output buffer (io)
boolean forwards;   // 0 for forwards, 1 for backwards (i)
* Reads characters from input, writes collation elements in output
void getCollationElements() {
    char c = input[inputPos++];
    int ce = collationElements[index[c>>8] + c&0xFF];
* Normally just returns ce. However, special forms indicate that
* the ce is actually an expansion, or that we have to search
* to see if the character was part of a contraction.
* Expansions use 
void processCE(int ce) {
    if (ce < 0xFFF00000) {
        output[outputPos++] = ce;
    } else if (ce >= 0xFFE00000) {
        copyExpansions(ce & 0x7FFFFF);
    } else {
        searchContractions(ce & 0x7FFFFF);
* Search through a contraction sublist to see if there is a match.
* Since the list is sorted, we can exit if our value is too high.<p>
* Since we have a length, we could implement this as a
* binary search, although we don't right now.<p>
* If we do find a match, we need to recurse. That's how "abc" would
* be handled.<p>
* If we fail, we return the non-matching case. That can be an expansion
* itself (it would never be a contraction).
void searchContractions(int offset) {
    if (forwards) inputPos++;
    else offset += input[inputPos++];
    short goal = (short)input[inputPos++];
    int limit = offset + contractionMatches[offset];
    for (int i = offset; i < limit; ++i) {
        short cc = contractionMatches[i];
        if (cc > goal) { // definitely failed
        } else if (cc == goal) { // found match
* Copy the expansion collation elements up to the terminator.
* Don't use 00000000 as a terminator, since that may be a valid CE.
* These elements don't recurse.
void copyExpansions (int offset) {
    int ce = expansions[offset++];
    while (ce != 0xFFFFFFFF) {
        output[outputPos++] = ce;
        ce = expansions[offset++];
* For random access, gets the start of a collation element.
* Any non-initial characters are in a sorted list, so
* we just check that list.<p>
* Since we have a length, we could implement this as a
* binary search, although we don't right now.
int getCollationElementStart(char[] buffer, int offset) {
    int i;
    for (i = offset; i > 0; --i) {
        char c = buffer[i];
        for (int j = 0; j < nonInitialsLength; ++j) {
            char n = nonInitials[j];
            if (c == n) continue main;
            if (c > n) break main;
    return i;

7 Weight Derivation

This section describes the generation of the Unicode Default Unicode Collation Element Table, and the assignment of weights to code points that are not explicitly mentioned in a Collation Element Table. This uses information from the Unicode Character Database on UnicodeData.txt (and documented in UnicodeData.html).

7.1 Derived Collation Elements

CJK Ideographs and Hangul Syllables are not explicitly mentioned in the default table. CJK ideographs are mapped to collation elements that are derived from their Unicode code point value as described in 7.1.3 Implicit Weights.

The collation algorithm requires that Hangul Syllables be decomposed. However, if the table is tailored so that the primary weights for Hangul Jamo (and all related characters) are adjusted, then the Hangul Syllables can be left as single code points and treated in the same way as CJK ideographs. That will provide a collation which is approximately the same as UCA, and may be sufficient in environments where individual jamo are not expected.

The adjustment is to move each initial jamo (and related characters) to have a primary weight corresponding to the first syllables starting with that jamo, and make all non-initial jamo (and related characters) be ignorable at a primary level.

7.1.1 Illegal code points

Certain code points are illegal in a data stream. These include non-characters (code points with the Noncharacter_Code_Point property in the Unicode Character Database), unpaired surrogates (code points with the General_Category property Cs), and out-of-range values (< 0 or > 10FFFF). Implementations may also choose to treat these as error conditions and respond appropriately, such as by throwing an exception.

If they are not treated as an error condition, they must be mapped to [.0000.0000.0000.], and thus ignored.

7.1.2 Legal code points

Any other legal code point that is not explicitly mentioned in the table is mapped a sequence of two collation elements as described in 7.1.3 Implicit Weights.

7.1.3 Implicit Weights

A character is mapped to an implicit weight in the following way. The result of this process consists of collation elements that are sorted in code point order, that do not collide with any explicit values in the table, and that can be placed anywhere (e.g. at BASE) with respect to the explicit collation element mappings (by default, they go after all explicit collation elements).

To derive the collation elements, the code point CP is separated into two parts, chosen for the correct numerical properties. First, separate off the top 6 bits of the code point. Since code points can go from 0 to 10FFFF, this will have values from 0 to 2116 (= 3310). Add this to the special value BASE.

AAAA = BASE + (CP >> 15);

Now take the bottom 15 bits of the code point. Turn the top bit on, so that the value is non-zero.

BBBB = (CP & 0x7FFF) | 0x8000;

The mapping given to CP is then given by:

CP => [.AAAA.0020.0002.][.BBBB.0000.0000.]

If a fourth or higher weights are used, then the same pattern is used: they are set to a non-zero value, etc. in the first collation element and zero in the second. (Since all distinct code points have different AAAA/BBBB combination, the exact values.)

The value for BASE depends on the type of character:

FB40 CJK Ideograph
FB80 CJK Ideograph Extension A/B
FBC0 Any other code point

These results make AAAA (in each case) larger than any explicit primary weight; thus the implicit weights will not collide with explicit weights. It is not generally necessary to tailor these values to be within the range of explicit weights. However if this is done, the explicit primary weights must be shifted so that none are between each of the BASE values and BASE + 34.

7.1.4 Trailing Weights

The range of primary weights from FC00 to FFFF are available for use as trailing weights especially for the case of Hangul Syllables. These syllables can be of the form LL*VV*T*: that is, one or more Lead jamo, followed by one or more Vowel jamo, followed optional by any number of Trail jamos. For more information, see Section 3.12 Conjoining Jamo Behavior.

Trailing weights are for characters that are given primary weights, but grouped as a unit together with a previous character, such as U+1160 HANGUL JUNGSEONG FILLER  through U+11F9 HANGUL JONGSEONG YEORINHIEUH. By tailoring these characters in this range, the units are ordered independent of subsequent characters with higher weights. Otherwise problems may occur, such as in the following example.

Case 1 Case 2
1 {G}{A}
2 {G}{A}{K}
2 {G}{A}{K}囗
1 {G}{A}囗

In this example, the symbols {G}, {A}, and {K} represent letters in a script where syllables (or other sequences of characters) are sorted as units. By proper choice of weights for the individual letters, the syllables can be ordered correctly. But the weights of the following letters may cause syllables of different lengths to change order. Thus {G}{A}{K} comes after GA in Case 1. But in Case 2, it comes before. That is, the order of these two syllables would be reversed when each is followed by a CJK character: in this case, U+56D7 (囗).

For Hangul syllables to sort correctly, either the UCA data must be tailored or the UCA algorithm (and data) must be tailored. The following are two possible solutions:

  1. Ensure that for all L, V, T: T < V < L, and

  2. One of the following options:

    1. Data

      1. Tailor each sequence of multiple L's that occurs in the repertoire as a contraction, with an independent primary weight after any prefix's weight, and

      2. Tailor the Vs and Ts to be Trailing Weights.

        • Note: Instead of tailoring the Vs and Ts to be Trailing Weights, one could tailor all Unicode characters that are otherwise higher than Vs and Ts to be lower than them.

    2. Algorithm. Add a terminator primary weight to the end of every standard Hangul syllable. This weight must be less than the primary weight for any Jamo character.

      • This can be done by adding the terminator between any pairs of characters that are not kept together according to the rules of Section 3.12.

For condition B.1.a, this means that if L1 has a primary weight of 555, and L2 has 559, then L1L1 would have to be given a weight from 556 to 558.

Note: If the repertoire of supported Hangul syllables is limited to modern syllables (those of the form LV or LVT), then the solutions are somewhat simpler: condition A and condition B.1.a can be dropped.

Note: The Unicode Consortium recognizes that one of these solutions should be implemented in the standard UCA algorithm and tables, but is attempting to work out a common approach to the problem with the ISO SC22 WG20 group, which takes considerable time. In the meantime, either of these approaches can be used for correct ordering.

7.2 Canonical Decompositions

Characters with canonical decompositions do not require mappings to collation elements, because Step 1.1 maps them to collation elements based upon their decompositions. However, they may be given mappings to collation elements anyway. The weights in those collation elements must be computed in such a way they will sort in the same relative location as if the characters were decomposed using Normalization Form D. By including these mappings, this allows an implementation handling a restricted repertoire of supported characters to compare strings correctly without performing the normalization in Step 1.1 of the algorithm.

A combining character sequence is called impeding if it contains any conjoining Jamo, or if it contains an L1-ignorable combining mark and there is some character that canonically decomposes to a sequence containing the same base character. For example, the sequence <a, cedilla> is an impediment, since cedilla is an L1-ignorable character, and there is some character, e.g. a-grave, that decomposes to a sequence containing the same base letter a. Note that although strings in Normalization Form C generally don't contain impeding sequences, there is nothing prohibiting them from containing them.

Note: Conformant implementations that do not support impeding character sequences as part of their repertoire can avoid performing Normalization Form D processing as part of collation.

7.3 Compatibility Decompositions

As remarked above, most characters with compatibility decompositions can have collation elements computed at runtime to save space, duplicating the work that was done to compute the Default Unicode Collation Element Table. This can be an important savings in memory space. The process works as follows.

1. Derive the decomposition. e.g.

2475 PARENTHESIZED DIGIT TWO => 0028, 0032, 0029

2. Get the CE for each character in the decomposition.

0028 [*023D.0020.0002] % LEFT PARENTHESIS
0032 [.06C8.0020.0002] % DIGIT TWO
0029 [*023E.0020.0002] % RIGHT PARENTHESIS

3. Set the first two L3 values to be lookup(L3), where the lookup uses the table in §7.3.1 Tertiary Weight Table. Set the remaining L3 values to MAX (which in the default table is 001F):

0028 [*023D.0020.0004] % LEFT PARENTHESIS
0032 [.06C8.0020.001F] % DIGIT TWO
0029 [*023E.0020.001F] % RIGHT PARENTHESIS

4. Concatenate the result to produce the sequence of collation elements that the character maps to.

2475 [*023D.0020.0004] [.06C8.0020.0004] [*023E.0020.0004] 

Some characters cannot be computed in this way. They must be filtered out of the default table and given specific values. An example is:


7.3.1 Tertiary Weight Table

Characters are given tertiary weights according to the following table. The Decomposition Type is from the Unicode Character Database. The Condition is either based on the General Category or on a specific list of characters. The weights are from MIN = 2 to MAX = 1F16, excluding 7, which is not used for historical reasons. The Samples show some minimal values that are distinguished by the different weights. All values are distinguished from MIN except for the katakana/hiragana values.

Type Condition Weight Samples
 NONE   0x0002 i ب ) mw 1⁄2 X
 <wide>   0x0003          
 <compat>   0x0004          
 <font>   0x0005 ℹ           
 <circle>   0x0006          
!unused!   0x0007            
 NONE Uppercase 0x0008 I     MW    
 <wide> Uppercase 0x0009        
 <compat> Uppercase 0x000A          
 <font> Uppercase 0x000B          
 <circle> Uppercase 0x000C          
 <small> small hiragana (3041, 3043,... 0x000D          
 NONE normal hiragana (3042, 3044, ...) 0x000E          
 <small> small katakana (30A1, 30A3,...) 0x000F        
 <narrow> small narrow katakana (FF67..FF6F) 0x0010          
 NONE normal katakana (30A2, 30A4, ...) 0x0011          
 <narrow> narrow katakana (FF71..FF9D),
narrow hangul (FFA0..FFDF)
 <circle> circled katakana (32D0..32FE) 0x0013          
 <super>   0x0014          
 <sub>   0x0015          
 <vertical>   0x0016          
 <initial>   0x0017          
 <medial>   0x0018          
 <final>   0x0019          
 <isolated>   0x001A          
 <noBreak>   0x001B            
 <square>   0x001C          
 <square> Uppercase 0x001D          
 <fraction>   0x001E         ½  
 n/a  (MAX value) 0x001F            

8 Searching and Matching (Informative)

The collation elements can also be used for matching string and for searching for strings, so that a proper native-language match is produced. For example, "ß" will properly match against "ss". Users of search algorithms should be allowed to modify the comparison strength, thus excluding differences at less significant levels. This is especially useful for searching, but can also apply to comparison.

Excluding differences at Level 3 has the effect of ignoring case and compatibility format distinctions between letters when searching. Excluding differences at Level 2 has the effect of ignoring accentual distinctions when searching.

Conceptually, a string matches some target where a substring of the target has the same sort key. But there are a number of complications:

  1. The lengths of matching strings may differ: "aa" and "å" would match in Danish.
  2. Because of ignorables (at different levels), the position where a string matches may be indefinite, depending on the attribute settings of the collation. For example, if hyphens are ignorable for a certain collation, then "abc" will match "abc", "abc-", "-abc-", etc.
  3. Suppose that the collator has contractions, and that a contraction spans the boundary of the match. Whether or not it is considered a match may depend on user settings, just as users are given a "Whole Words" option in searching. So in a language where "ch" is a contraction, "bac" would not match in "bach" (given the proper user setting).
  4. Similarly, combining character sequences may need to be taken into account. Users may not want a search for "abc" to match in "...abç...". However, this may also depend on language and user customization.
  5. The above two conditions can be considered part of a general condition: "Whole Grapheme Clusters Only"; though probably expressed in user interfaces with more natural wording as "Whole Characters Only". This is very similar to the common "Whole Words Only" checkbox that is included in most search dialog boxes. (For more information on grapheme clusters, see UTR #18: Unicode Regular Expression Guidelines)
  6. Certain Thai and Lao vowels are swapped with the preceding character. For example, the text string “เข” (...\u0E40\u0E02...) is modified internally in collation to “ขเ” (...\u0E02\u0E40...). This may mean that a string logically matches a discontiguous section of another string. If, however, the vowels are considered to be part of a grapheme cluster, then this situation is handled by the "whole grapheme clusters only" option.
  7. If the matching is does not check for "Whole Grapheme Clusters Only", then some other complications may occur. For example, suppose that P is "x^", and Q is "x ^¸". Because the cedilla and circumflex can be written in arbitrary order and still be equivalent, one would expect to find a match for P in Q. A canonically-equivalent matching process requires special processing at the boundaries to check for situations like this. (It does not require such special processing within the P or the substring of Q since collation is defined to observe canonical equivalence.)

The following definitions come into play:

DS1. Define S[start,end] to be the substring of S that includes the character after the offset start up to the character before offset end. For example, if S is "abcd", then S[1,3] is "bc".

Suppose there is a collation C, a pattern string P and a target string Q. C has some particular set of attributes, such as a strength setting, and choice of variable weighting.

DS2. There is a match according to C for P within Q[s,e] if and only if C generates the same sort key for P as for Q[s,e].

DS3. There is a canonical match according to C for P within Q[s,e] if and only if there is some Q', canonically equivalent to Q[s,e], and some s' and e' such that P matches within Q[s',e'].

DS4. The match is minimal if for all positive i and j, there is no match at Q[s+i,e-j]. In such a case, we also say that P matchs at Q[s,e].

DS5. The match is grapheme-complete if s and e are both at grapheme cluster boundaries.

DS6. The first forward match for P in Q starting at b is the least offset s greater than or equal to b such that for some e, P matches within Q[s,e].

DS7. The first backward match for P in Q starting at b is the greatest offset e less than or equal to b such that for some s, P matches within Q[s,e].


Thanks to Åke Persson, Kent Karlsson, Roozbeh Pournader, Vladimir Weinstein, and Richard Gillam for their feedback on previous versions of this document, and Cathy Wissink for her contributions to the text.



The latest version of this file is:
The version at the time of this publication is:
Note: ftp access is available, starting at:

[CanStd] CAN/CSA 2243.4.1
[FAQ] Unicode Frequently Asked Questions
For answers to common questions on technical issues.
[Feedback] Reporting Errors and Requesting Information Online
[Glossary] Unicode Glossary
For explanations of terminology used in this and other documents.
[ICUCollator] http://oss.software.ibm.com/icu/userguide/Collate_Intro.html
[JavaCollator] http://java.sun.com/j2se/1.4/docs/api/java/text/Collator.html,
[LDML] Locale Data Markup Language Specification
Hosted on http://oss.software.ibm.com/cvs/icu/locale/ldml-spec.html
Also see: http://www.openi18n.org/subgroups/lade/locale/
[Reports] Unicode Technical Reports
For information on the status and development process for technical reports, and for a list of technical reports.
[Sample] http://www.unicode.org/reports/tr10/Sample/
[SoStd] ISO/IEC 14651
[Test] The latest version of these files are:
The versions at the time of this publication are:
Note: ftp access is available, starting at:
[Unicode] The Unicode Consortium. The Unicode Standard, Version 4.0. Reading, MA, Addison-Wesley, 2003. 0-321-18578-1.
[Versions] Versions of the Unicode Standard
For details on the precise contents of each version of the Unicode Standard, and how to cite them.


The following summarizes modifications from the previous version of this document.

  • Changed the version to synchronize with versions of the Unicode Standard, so that the repertoire of characters is the same. This affects the header and C4.
  • Added new Introduction. This covers concepts in Version 3.0, Section 5.17, but is completely reworked. The Scope section has been recast and is now at the end of the introduction.
  • In 6.9 Tailoring Example: Java, added informative reference to LDML; moved informative reference to ICU.
  • Added explanation of different ways that the Hangul problem can be solved in 7.1.4 Trailing Weights.
  • Copied sentence from Scope up to Summary, for more visibility.