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5.3.1 Motion Quanta
-------------------

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.

 -- Register: \n[.H]
 -- Register: \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.  *Note Drawing Geometric Objects::.

     .tm \n[.H]
         error-> 24
     .nf
     \l'36u' 36u
     \l'37u' 37u
         => _ 36u
         => __ 37u