Re: device-dependent warnings

[Dave Kemper]

On 5/10/23, Dave Kemper <saint.snit@gmail.com> wrote:
> Nonintuitively, .ss's units aren't in fractions of standard
> typographical measurements, but 1/12 of the current font's ordinary
> space.

And I just learned (or maybe relearned) this is a deviation from AT&T
troff's .ss units, which are a fixed 1/36 em.

This difference is quasi-documented in the current groff_diff(7): that
page does say that the "units are twelfths of the space width of the
current font," and, although it doesn't explicitly state this is a
change from AT&T troff, the sentence being in groff_diff(7) might
suggest this.

On the other hand, this sentence appears in a section called "Extended
requests," and the primary point of the .ss entry here is to note that
groff gives it a second argument, which IS an extension of its
historical implementation, whereas the change in units can't
reasonably be deemed "extending" the request.  And as the second
argument is new, a reader would expect its units to be documented, so
mentioning that they're also the same as the first argument's units
could plausibly be read as a courtesy or a phrasing efficiency rather
than as documenting a change from historical behavior.

So groff_diff(7) might benefit from clarifying this difference.
(Although the wording was vastly overhauled from 1.22.4's
groff_diff(7), all the above points apply there as well, so this isn't
a documentation regression.)  It might also help readers to state here
what AT&T troff's .ss units were.