The original Ossanna troff generates proprietary printer codes for the
Wang C/A/T Phototypesetter; we'll call this "CAT Troff".  This version
comes bundled in many systems, in particular, SunOS, most other
Berkeley-derived systems, some SVR4s and is contained in the Xenix Text
Processing package.  The AT&T, BSD and Xenix variants all differ
slightly, but not in any important ways.  CAT troff is useless without
filters to convert the "CAT codes" to something else, especially since
the C/A/T is as common as a California condor.  These filters are
described later.

In 1981, Brian Kernighan of Bell Laboratories rewrote troff to generate
a generic typesetting language.  These troffs are called "ditroff" (for
device-independent troff) and contain some additional features such as
arbitrary line drawing, and more flexible font handling.  "Documenter's
Workbench" (DWB) is a package containing ditroff and several other
typesetting filters.  The latest AT&T version is DWB 3.4.1, which sold as
source code; most commercial binary variants (Elan, SoftQuad, Image
Network, etc.) are based on DWB 2.0.  The Free Software Foundation
distributes a re-engineered version of ditroff called groff.

If you have a troff and want to know which it is, type:
	troff < /dev/null > /dev/null
If it responds with "typesetter busy" or "No /dev/cat; use -t or -a",
you have CAT troff, otherwise it's ditroff.  If you get an answer from:
	dwbv
you have the AT&T release 3.0 or later.  The differences are too
numerous and subtle to document here, but the variants are about
95% compatible.  All are ASCII based, but DWB 3.4, Groff and
AIX 3.2's troff also accept ISO 8859-1 (aka ISO Latin 1), the
Western European character set and are 8 bit clean.  Any 8-bit
clean ditroff can be reconfigured to support alternate 8859-x
character sets.  AIX 3.2 ditroff also supports Kanji (multi-byte)
to a certain extent (as per IBM-932).

See TR16 for more on DWB.