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Posted by Jukka K. Korpela on 05/30/07 10:58
Scripsit Toby A Inkster:
> Ben C wrote:
>
>> Technically it's a measure of width (of an 'M') rather than of
>> height.
>
> In traditional typography, yes.
No, the em unit was originally related to the square in which inscription
letters were designed, so it related both to the width and to the height of
"M", but the connection was lost almost two thousand years ago.
> The font height itself is quite a wishy-washy concept,
Rather, it is redundant. It is meant to help people to get a rough idea:
> but it can
> normally be thought of as the height of most capital letters and
> lower-case letters with ascenders (e.g. 'b', 'd', 'f', etc.).
No, their height is considerably smaller than the font size ("font height").
Try it. Use a 1em by 1em element with a border, with line-height: 1, with a
single character inside it, and see how much space there is both above and
below any of those characters.
If you take the distance from the lowest descender up to the highest
ascender, you get close. Few characters have both a descender and an
ascender; lowercase ("small") thorn is one of the few exceptions. But you
really need a diacritic on it, too, e.g. þ́ (thorn with acute,
not used in any human language) to get a character that occupies the full
height of the font - roughly speaking.
In fact, ascenders, descenders, and diacritic marks may extend a bit past
the limits set by the "font height".
The font size, or "font height", is really a typographic design concept, as
the CSS specifications tell us. When a typographer starts designing a font,
he uses an em square as a tool, designing characters so that they
comfortably fit into it, and some font designs use a larger part of the
square than others, and use it differently. Any normal character is expected
to remain within the em square boundaries, or at most touch them or cross
them just a little.
In a sense, the em square is the drawing board of a font designer. How he
uses it is up to him
--
Jukka K. Korpela ("Yucca")
http://www.cs.tut.fi/~jkorpela/
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