Numbers can have scale indicators suffixed to them. The suffixes mean:
Basic units in troff nroff i Inch 432 240 c Centimeter 170 94 p Pica (1/6 inch) 72 40 m Em (S points) 6 S C n En (1/2 Em) 3 S C p Point (1/72 inch) 6 3 u Basic unit 1 1 v Vertical line space V V none Default, see belowWhere
Certain requests have default scales; all others ignore scale entirely. If a number argument to a request that expects a scaled number doesn't have a suffix, it is multiplied by the default scale. That holds for number register accesses as well; if they already are in basic units, they should be suffixed with a u.
| positionis the distance between the current position and the given position, vertically (when used in a vertical request) or horizontally (for all others.) The result is in basic units.
In numerical expressions, troff understands +, -, *, /, %, <, >, <=, >=, = or ==; additionally, & is and and : is or. There is no operator precedence; evaluation proceeds strictly left-to-right. Use parentheses.
The multiplication implied by default scales applies to every single number in an expression; if you don't want it multiplied, suffix a u.