An output device’s basic unit u is not necessarily its smallest
addressable length; u can be smaller to avoid problems with
integer roundoff.  The minimum distances that a device can work with in
the horizontal and vertical directions are termed its motion
quanta.  Measurements are rounded to applicable motion quanta.
Half-quantum fractions round toward zero.
\n[.H] ¶\n[.V] ¶These read-only registers interpolate the horizontal and vertical motion quanta, 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