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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 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 drawing position;
page advancement;
drawing of geometric objects;
and device control commands,
a catch-all for other operations.
The last includes directives to start and stop output,
identify the intended output device,
and embed URL hyperlinks in supported output formats.