On 5/10/23, Dave Kemper <saint.s...@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.
