User Guide

Hinting
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But even if the middle delta command is set on an untouched point it is
applied to the point before final interpolation of the untouched points. This
point becomes touched after interpretation of the middle delta command
so the position of other untouched points will be changed proportionally to
the shift of that point:
Middle delta instructions
We recommend using the middle delta command with untouched points
when it’s necessary to shift several points at once. Usually you have such a
situation on diagonal and curved stems