For the sake of old phototypesetters and compatibility with old versions
gtroff has the concept of font positions,
on which various fonts are mounted.
Mount font font at position pos (which must be a
non-negative integer). This numeric position can then be referred to
with font changing commands. When
gtroff starts it is using font
position 1 (which must exist; position 0 is unused usually at
The current font in use, as a font position, is available in the read-only number register ‘.f’. This can be useful to remember the current font for later recall. It is associated with the current environment (see Environments).
.nr save-font \n[.f] .ft B ... text text text ... .ft \n[save-font]
The number of the next free font position is available in the read-only number register ‘.fp’. This is useful when mounting a new font, like so:
.fp \n[.fp] NEATOFONT
Fonts not listed in the DESC file are automatically mounted on
the next available font position when they are referenced. If a font is
to be mounted explicitly with the
fp request on an unused font
position, it should be mounted on the first unused font position, which
can be found in the
.fp register. Although
not enforce this strictly, it is not allowed to mount a font at a
position whose number is much greater (approx. 1000 positions) than
that of any currently used position.
fp request has an optional third argument. This argument
gives the external name of the font, which is used for finding the font
description file. The second argument gives the internal name of the
font, which is used to refer to the font in
gtroff after it has
been mounted. If there is no third argument then the internal name is
used as the external name. This feature makes it possible to use fonts
with long names in compatibility mode.
ft request and the
\f escape have alternative
syntax forms to access font positions.
Change the current font position to nnn (one-digit position n, two-digit position nn), which must be a non-negative integer.
If nnn is associated with a style (as set with the
request or with the
styles command in the DESC file), use
it within the current font family (as set with the
\F escape, or the
family command in the
this is font 1 .ft 2 this is font 2 .ft \" switch back to font 1 .ft 3 this is font 3 .ft this is font 1 again
See Changing Fonts, for the standard syntax form.