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The basic unit
u
is not necessarily an output device’s smallest addressable length;
u
can be smaller to avoid integer rounding errors.
The minimum distances that a device can work with
in the horizontal and vertical directions are termed its
motion quanta.
The formatter rounds measurements to applicable motion quanta.
Half-quantum fractions round toward zero.
These read-only registers interpolate the horizontal and vertical motion quantum, respectively, of the output device in basic units.
For example, we might draw short baseline rules on a terminal device as follows. See Drawing Geometric Objects.
.tm \n[.H] error→ 24 .nf \l'36u' 36u \l'37u' 37u ⇒ _ 36u ⇒ __ 37u