The intermediate output language of AT&T troff was
first documented in A Typesetter-independent TROFF, by Brian
Kernighan, and by 1992 the AT&T troff manual was
updated to incorprate a description of it.
The GNU troff intermediate output format is compatible with this
specification except for the following features.
groff devices are also fundamentally different from the ones
in AT&T troff. For example, the AT&T
PostScript device is called post and has a resolution of only 720
units per inch, suitable for printers 20 years ago, while groff’s
ps device has a resolution of 72000 units per inch. Maybe, by
implementing some rescaling mechanism similar to the classical
quasi-device independence, groff could emulate AT&T’s
post device.
gtroff, while
AT&T troff has point (‘p’). This isn’t an
incompatibility but a compatible extension, for both units coincide for
all devices without a sizescale parameter in the DESC
file, including all postprocessors from AT&T and
groff’s text devices. The few groff devices with a
sizescale parameter either do not exist for AT&T
troff, have a different name, or seem to have a different
resolution. So conflicts are very unlikely.
gtroff used this
feature it is kept for compatibility reasons.