File: groff.info,  Node: Default Units,  Prev: Motion Quanta,  Up: Measurements

5.3.2 Default Units
-------------------

A general-purpose register (one created or updated with the 'nr'
request; see *note Registers::) is implicitly dimensionless, or reckoned
in basic units if interpreted in a measurement context.  But it is
convenient for many requests and escape sequences to infer a scaling
unit for an argument if none is specified.  An explicit scaling unit
(not after a closing parenthesis) can override an undesirable default.
Effectively, the default unit is suffixed to the expression if a scaling
unit is not already present.  GNU 'troff''s use of integer arithmetic
should also be kept in mind (*note Numeric Expressions::).

   The 'll' request interprets its argument in ems by default.  Consider
several attempts to set a line length of 3.5 inches when the type size
is 10 points on a terminal device with a resolution of 240 basic units
and horizontal motion quantum of 24.  Some expressions become zero; the
request clamps them to that quantum.

     .ll 3.5i      \" 3.5i (= 840u)
     .ll 7/2       \" 7u/2u -> 3u -> 3m -> 0, clamped to 24u
     .ll (7 / 2)u  \" 7u/2u -> as above
     .ll 7/2i      \" 7u/2i -> 7u/480u -> 0 -> as above
     .ll 7i/2      \" 7i/2u -> 1680u/2m -> 1680u/24u -> 35u
     .ll 7i/2u     \" 3.5i (= 840u)

The safest way to specify measurements is to attach a scaling unit.  To
multiply or divide by a dimensionless quantity, use 'u' as its scaling
unit.