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.
Murray
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.