L2/02-035

To: | UTC |

Re: | Definition of Canonical Composite |

From: | Mark Davis |

Date: | 2001-01-23 |

I propose that we make the following addition to the end of 3.6 Decomposition (revision) in Unicode 3.2.

*Add the following text after D23:*

D23a *Canonical composite*: a character which is not identical to its
canonical decomposition.

- For example: U+00E0 LATIN SMALL LETTER A WITH GRAVE is a canonical
composite, since its canonical decomposition is to the two characters:
*a*and*grave*. U+212A KELVIN SIGN is a canonical composite since its canonical decomposition is to a*K*. - Using D21 and D23a, all characters can thus be broken down into the
following four categories:

Category Examples non-composite **a**,**b**,**c**, ...canonical composite *a-grave*(**à**),*kelvin sign*(**K**), ...compatibility composite *micro-sign*(**µ**),*square mil*(**㏕**), ...canonical & compatibility composite *greek upsilon with acute and hook symbol*(**ϓ**), ...

We define *decomposable* character in D18. We say that it is equivalent
to one or more characters according to the decomposition mappings. We give *composite*
and *precomposed* as aliases for this term. However, we do not, in that
definition, distinguish between the kinds of decompositions that determine a
decomposible/composite.

To resolve an ambiguity in our terminology, in 3.2 we are using the term *compatibility
composite* in D21.
That distinguishes a particular subset of composites, based on the kind of
decomposition. The term *canonical composite* distinguishes a different
subset of composites, based upon the other kind of decomposition. While it is in
some sense is a natural fallout from the definitions of *canonical* and *composite*,
it is important enough that we should have a formal definition.

People too easily misuse the term *composite* (or *precomposed*) to
mean *only* canonical composite -- I've seen this on a number of occasions.
A formal definition -- and inclusion of the table with examples -- will help to
reduce that confusion.