The fundamental operation of the GNU
troff formatter is the translation of the
groff
input language
into a series of instructions concerned primarily
with placing glyphs or geometric objects
at specific positions on a rectangular page.
In the following discussion,
the term
command
always refers to this device-independent output language,
and never to the
groff
language intended for direct use by document authors.
Device-independent output commands comprise several categories:
glyph output;
font,
color,
and text size selection;
motion of the printing position;
page advancement;
drawing of geometric objects;
and device control commands,
a catch-all for operations not easily classified
as any of the foregoing,
such as directives to start and stop output,
identify the intended output device,
or place URL hyperlinks in supported output formats.