Bottom margin vs. descenders question

[barbara beeton]

On Wed, 24 Jun 2020, Doug McKenna wrote:

> Dear typesetting cogniscenti,
>
> In "official typesetting rules" (TeX or otherwise) are descenders supposed to be completely above the bottom margin bounds of a page?  Or is just the baseline of the bottom line of a full page supposed to conform to the margin bounds?

Peter Flynn has brought up metal, so let's look at that.  In metal
fonts, all type slugs of the same font have the same dimensions in
respect to height as measured on the printed surface; while the
printing surface has gaps at the top and bottom of characters to
yield an even baseline.

Ignoring math displays, when two facing pages are viewed side by side,
the bottom baselines are expected to line up.  Occasional descenders
may extend below the baseline (into the margin, if you will).  This
is the model followed by TeX.

An even more rigid model followed by traditional typesetting is "grid
typesetting", in which *all* baselines line up on facing pages.  As has
often been noted, this is *not* followed by, and is not easy to obtain
with TeX, especially in the presence of math displays.

I can't say that this is a "rule", but it is certainly traditional
practice.
 						-- bb