Re: Widths in tables

Murray Maloney (murray@sco.COM)
Mon, 24 Apr 95 19:15:40 EDT

Jon Bosak writes:
> [Bert Bos:]
> > Dave Ragget writes:
> [stuff omitted]
> > |The use of ems is problematic in this case as an em isn't the same
> > |as the width of a character for a fixed pitch font. My understanding
> > |is that the em is equal to the point size which is related to the
> > |height of the font - not its width.
> >
> > That is not the way I heard it. em is a unit of width, not of height
> > (ex and others are for height). It is a measure associated with a font
> > by the designer and it is traditionally about the width of an M (hence
> > the name). An en is exaclty half an em.
> Dave is correct. An em is the body height of the font. If the font
> is a 12 point font, then the body height is 12 points (that's what "12
> point font" means) and the value of an em is 12 points. In *most*
> traditional latin fonts, the lower case and capital "m" characters (I
> mean the physical pieces of metal) are exactly as wide as they are
> tall, making the type itself square in cross section; that's why ems
> are often called "em quads".
Well, everybody is right to a certain extent. The measure of an em
is certainly a characteristic of the height of the font. But the
em is more often used as a measuer of width -- like an em space
or an em dash. To quote from material that I once wrote
in a book for SoftQuad:

In older technologies, when characters were beveled
onto pieces of metal, the point size measured the
piece of metal. While electronic typesetting is
no longer concerned with these pieces of metal,
this method of measurement is preserved by consideration
of an imaginary bounding box in which the letter is
contained. The point size of of a piece of type
describes the height of this imaginary bounding box
-- not the measured size of characters on paper.

This paragraph is -- in the book -- accompanied by a figure
which depicts the various terms that are used in character
measuerment (set width, ascender, descender, x-height and
point size. It is instructive to note that the point size
of a font typically includes a small margin of white space
above the ascender and below the descender of the tallest
and lowest characters respectively. Thus, it is possible
for a superscript or subscript to extend beyond the
ascender or descender and yet remain within the point size
of the font.


P.S. My sources for research included "The Chicago Manual
of Style", "Words into Type", "Pocket Pal: A Graphic Arts
Production Handbook", "Typography: Design and Practice",
"The Thames and Hudson Manual of Typography" and
"Designing with Type: a Basic Course in Typography".

These are all recommended reading or reference material
for those interested in working with paper or electronic
forms of typesetting.