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Re: What's an em (was RE: Units, font sizing, and zoom suggestion for CSS 3)

From: Tiro Typeworks <tiro@tiro.com>
Date: Mon, 31 Jan 2000 13:50:37 -0500 (EST)
Message-ID: <Pine.NEB.4.03.10001311030060.21905-100000@shell.portal.ca>
To: Karlsson Kent - keka <keka@im.se>
cc: www-style@w3.org, "'www-font@w3.org'" <www-font@w3.org>
While I'm not about to get into a discussion os CSS, about which I know
very little, I would like to point out that the term 'em' has been used in
English language typography for 250+ years to refer to a horizontal unit
of measure equal to the body height of type at a given point size. Hence,
16pt type has an em width of 16 points. It is in this sense that the term
is still used by font developers although, obviously, digital glyphs are
no longer strictly bound by the confines of the body height.

Although the term em derives from the pronunciation of the letter M, it
was never traditionally used to refer to the width of that letter, which
would vary considerably, of course, from typeface to typeface and from
font to font. This is attested to by the fact that individual metal fonts
did not come with separately cast metal slugs equal in width to the letter
M; rather, compositors would have cases of standard em and en slugs for
each point size.

Increasingly, in digital font development, the term em is also used to
refer to the scaleable body height; such that, in TrueType font
development, we speak of a 2048 unit em, and in Type 1 PS of a 1000 unit
em, referring to the most common design grids of these font formats.

I leave it to you to figure out how this relates to CSS, but if TEX and
any other system is using the term em to refer to the width of the letter
M, this is due to a misunderstanding.

John Hudson

Tiro Typeworks
Vancouver, BC
Received on Monday, 31 January 2000 17:50:56 UTC

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