RE: Difference between EM QUAD and EM SPACE

From: Kenneth Whistler (
Date: Fri Jul 07 2000 - 19:28:51 EDT

Donald Figge noted:

> An EM QUAD is a square [space] with sides equal to the point size of the
> font. An EM SPACE is a horizontal measure, usually equal to the side of an
> em quad. In condensed fonts, the em space is sometimes smaller than that,
> for aesthetic and readability reasons. In expanded fonts, the em space may
> be wider than the standard, for the same reasons.
> Donald Figge
> /

*sigh* I guess no question is an easy one, is it?

Let me quote from XCCS (Xerox Character Code Standard), 1980 edition
(the one which is the relevant source standard for Unicode 1.0):

"For a given size of type, an "em" is a unit of width equal to the type font.
For example, in a 12-point font, an em is 12 points high and 12 points wide.
The space characters provided by the standard have widths expressed in units
of ems; their width is, therefore, proportional to the size of the font used."
                                                  -- p. C-8

Then on p. C-9 is the table of spaces, the relevant section of which is:

Code(s) Name Also called Width
357 | 55 em quad em space 1.0 em fixed width
357 | 54 en quad en space 1/2 em fixed width

Clearly, the XCCS standard treated these as synonyms, and allocated one
code point for the em and one code point for the en.

Now from Robert Bringhurst, The Elements of Typographic Style, Second
Edition. 1996. p. 291:

"Em In linear measure, a distance equal to the type size, and in square
     measure, the square of the type size. Thus an em is 12 pt (or a 12 pt
     square) in 12 pt type, and 11 pt (or an 11 pt square) in 11 pt type.
     Also called mutton."

and p. 295:

"Quad An em. Also called mutton quad."

So what we have here is a confusion of the two types of type measurement --
linear measurement and square measurement -- with the need for encoding
two separate *characters* for the relevant spaces to represent an em
or an en.

Clearly there is (and was) no need for separate *characters* to indicate
an em measured linearly and an em measured square. The encoding in
Unicode 1.0 was simply a duplication that was not caught at the time,
but which was "rectified" later by the means of designating canonical
equivalences for the duplicated characters.

There is no particular need, as I see it, to reify an unneeded *character*
difference after the fact, simply because the two differently named
characters got into the standard by mistake.

Another installment in the ongoing saga: Every Character Has a Story.


P.S. Donald Figge is right about the non-fixed nature of the 'fixed width'
em (or en). Em and en measurement is adjusted by the expansion factor
of expanded (or condensed) fonts.

> -----Original Message-----
> From: Patrick Andries []
> Sent: Friday, July 07, 2000 3:32 PM
> To: Unicode List
> Subject: Difference between EM QUAD and EM SPACE
> Could someone explain to me what is the difference between an EM QUAD and an
> Patrick Andries
> Dorval (Québec)

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