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(groff)Troff and Nroff Mode

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Troff and Nroff Mode
====================

   Originally, `nroff' and `troff' were two separate programs, the
former for tty output, the latter for everything else.  With GNU
`troff', both programs are merged into one executable, sending its
output to a device driver (`grotty' for tty devices, `grops' for
POSTSCRIPT, etc.) which interprets the intermediate output of `gtroff'.
For UNIX `troff' it makes sense to talk about "Nroff mode" and "Troff
mode" since the differences are hardcoded.  For GNU `troff', this
distinction is not appropriate because `gtroff' simply takes the
information given in the font files for a particular device without
handling requests specially if a tty output device is used.

   Usually, a macro package can be used with all output devices.
Nevertheless, it is sometimes necessary to make a distinction between
tty and non-tty devices: `gtroff' provides two built-in conditions `n'
and `t' for the `if', `ie', and `while' requests to decide whether
`gtroff' shall behave like `nroff' or like `troff'.

 - Request: .troff
     Make the `t' built-in condition true (and the `n' built-in
     condition false) for `if', `ie', and `while' conditional requests.
     This is the default if `gtroff' (_not_ `groff') is started with
     the `-R' switch to avoid loading of the start-up files `troffrc'
     and `troffrc-end'.  Without `-R', `gtroff' stays in troff mode if
     the output device is not a tty (e.g. `ps').

 - Request: .nroff
     Make the `n' built-in condition true (and the `t' built-in
     condition false) for `if', `ie', and `while' conditional requests.
     This is the default if `gtroff' uses a tty output device; the
     code for switching to nroff mode is in the file `tty.tmac' which
     is loaded by the start-up file `troffrc'.

   Note: Conditionals and Loops, for more details on built-in
conditions.

   For tty output devices, underlining is done by emitting sequences of
`_' and `\b' (the backspace character) before the actual character.
Literally, this is printing an underline character, then moving back
one character position, and printing the actual character at the same
position as the underline character (similar to a typewriter).
Usually, a modern terminal can't interpret this (and the original
Teletype machines for which this sequence was appropriate are no longer
in use).  You need a pager program like `less' which translates this
into ISO 6429 SGR sequences to control terminals.