In AT&T troff output, the writing of a single glyph is
mostly done by a very strange command that combines a horizontal move
and a single character giving the glyph name. It doesn’t have a command
code, but is represented by a 3-character argument consisting of exactly
2 digits and a character.
Move right dd (exactly two decimal digits) basic units ‘u’, then print glyph g (represented as a single character).
In GNU troff, arbitrary syntactical space around and within this
command is allowed. Only when a preceding command on the same line ends
with an argument of variable length is a separating space obligatory.
In AT&T troff, large clusters of these and other
commands are used, mostly without spaces; this made such output almost
unreadable.
For modern high-resolution devices, this command does not make sense
because the width of the glyphs can become much larger than two decimal
digits. In gtroff, this is only used for the devices X75,
X75-12, X100, and X100-12. For other devices, the
commands ‘t’ and ‘u’ provide a better functionality.