Posted by Ben C on 05/31/07 22:17

On 2007-05-31, Jukka K. Korpela <jkorpela@cs.tut.fi> wrote:
> Scripsit Bernhard Sturm:
>
>>>> Technically it's a measure of width (of an 'M') rather than of
>>>> height.
>>>
>>> No it isn't.
> - -
>> Jukka: this is true in it's application, but Ben was talking about
>> it's 'technical' origin.
>
> No, as you can see from your own quotation, Ben claimed that the em _is_
> "technically" a measure of width. That's plain wrong.

I accept that. What did it for me was the idea of an "M square".
Although I suppose that makes saying it's a measurement of width moot
rather than wrong. But since I was correcting someone for saying it was
a measurement of height I can't really pretend that's what I meant.

Do we know whether Roman M-squares were actually equilateral?

> A common misconception, not a mortal sin,

Thank goodness for that.

Various websites do say it's a measure of width, e.g.
http://www.fontfactory.com/glossary.php. So I reserve some judgment on
the meaning in its wider context. But Mr Korpela makes a good point
about what it means in CSS.

I find this in CSS 2:

http://www.w3.org/TR/REC-CSS2/fonts.html#emsq

15.4.3

Certain values, such as width metrics, are expressed in units that
are relative to an abstract square whose height is the intended
distance between lines of type in the same type size. This square is
called the em square and it is the design grid on which the glyph
outlines are defined.

I missed that because I usually only work with CSS 2.1, from which all
that has been removed and replaced with just a vague reference to "a
concept used in typography".

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