Re: "em" should be horizontal, "ex" vertical

Peter Flynn (pflynn@imbolc.ucc.ie)
12 Aug 1997 01:33:31 +0100


Date: 12 Aug 1997 01:33:31 +0100
From: Peter Flynn <pflynn@imbolc.ucc.ie>
In-reply-to: <199708111255.OAA26052@stovner.a.sol.no> (message from Hakon Lie
To: howcome@w3.org
Cc: www-html@w3.org
Message-id: <199708120033.BAA27261@imbolc.ucc.ie>
Subject: Re: "em" should be horizontal, "ex" vertical

    > It _was_ once the width of an M, at some time in the past
    > when an M was assumed to be as wide as it was high.

h&kon writes:

   do you know when and why (technolgy?) this changed? Also, since the
   body size includes the descender (which "M" doesn't have) this
   change increased the size of an em, right?

I haven't managed to find a direct reference, but I think Moxon's
_Mechanick Exercises_ (1680) refers to ems, so they were established
as a measure at that stage (someone on TYPO-L can confirm or deny my
recollection, I'm sure). By that time, printing with Roman type was
common, so an em would have been related to an M as we know it, rather
than to the wider and more decorative blackletter capitals of earlier
periods. A brief look at some specimen sheets confirms that a Roman M
is nevertheless usually wider than it is high, so this compensates for
the lack of a descender. Long before the invention of mechanical
composition, an em was well established as the square of the current
point size, and a pica em as 12 points.

Historical irrelevancy: in noisy mechanical composing rooms, an em was
referred to as a "mutton" and an en (6pt) as a "nut", it being easier
to distinguish these two words by lip movement than to hear the
difference between "em" and "en".

///Peter