gtroff (like many other programs) requires numeric parameters to
specify various measurements. Most numeric parameters1 may have a
measurement unit attached. These units are specified as a single
character which immediately follows the number or expression. Each of
these units are understood, by
gtroff, to be a multiple of its
basic unit. So, whenever a different measurement unit is
gtroff converts this into its basic units. This
basic unit, represented by a ‘u’, is a device dependent measurement
which is quite small, ranging from 1/75th to 1/72000th of an
inch. The values may be given as fractional numbers; however,
fractional basic units are always rounded to integers.
Some of the measurement units are completely independent of any of the
current settings (e.g. type size) of
The other measurements understood by
gtroff depend on settings
currently in effect in
gtroff. These are very useful for
specifying measurements which should look proper with any size of text.
groff, this is half of an em.
 those that specify vertical or horizontal motion or a type size