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EM dashes (was: What's an em)

From: Jelks Cabaniss <jelks@jelks.nu>
Date: Tue, 1 Feb 2000 20:30:09 -0500
To: <www-style@w3.org>, <www-font@w3.org>
Message-ID: <NBBBICMNIPCICMKJECCBKEGMDFAA.jelks@jelks.nu>
No matter what TeX says, classical typography's definition of an em-square or
em-space has always been a square or space the specified size of the font, not
the width or height of the letter M.

TrueType (in general) echoes this.  Using MS Word, I printed several 72 point Ms
and em-dashes.  The results when measured:

72pt Times Roman -
	M height: 48pt
	M  width: 61pt
	em-dash : 72pt

72pt Arial -
	M height: 51pt
	M  width: 50pt
	em-dash : 72pt

In other words, the width of the em-dash is exactly the width of the font-size
specified.  This makes sense, and is the conventional way of doing it.  In metal
and phototypesetting, if you hit the "EM Space" key, you get a space exact X
points wide -- where X is the specified point size.  (Paragraphs in books and
newspapers generally had a text-indent of one em-space, though sometimes it
could be an em-space + an en-space, or even 2 or more em-spaces.).

But the newer "web" fonts have a different idea of em-dash widths:

72pt Georgia -
	M height: 50pt
	M  width: 61pt
	em-dash : 54pt

72pt Verdana -
	M height: 52pt
	M  width: 47pt
	em-dash : 60pt

I don't know where this is coming from.  Are em-dashes (and, by extension
em-spaces) now of variable width?

/Jelks
Received on Tuesday, 1 February 2000 20:32:10 GMT

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