[texhax] Em width (Was re: Overfull boxes)

[Philip TAYLOR]

Joel C. Salomon wrote:

>
> I stand corrected. But—
>
> What's going on with the example you gave? Why is this so?

Without doing any research (always a bad idea : I am usually
proved wrong) I would postulate that when I specify (e.g.,)

	\font \xviiirm = cmr10 at 18 true pt

I am specifying (in a very notional sense) the /height/ of
the font (which is why one so frequently sees font sizes paired
with leading, as in 10/12 pt Times Roman.).  However, when
I ask for the size of the em unit in the current font, I am
asking for a width.  Unless the font is square, the two
dimensions are unlikely to be the same, and the more the
font is extended or compressed, the less likely it is that
width and height will be close to each other.  This
argument fails for CSS, but there the authors have deliberately
chosen to place expedience over typographic history; fortunately
Knuth was more respectful of the traditions and usages of
countless generations of typographers and typesetters.

** Phil.