Ascending melodic minor raises 6 and 7; descending reverts to natural minor's 6 and 7

The 6th/7th scale degrees are raised when the line ascends stepwise through them, and revert to natural-minor spelling when descending stepwise; harmonic minor's raised 7th applies regardless of direction when functioning as a leading tone.

An ascending A-minor run has the raised 6th spelled Gb4 instead of F#4, or a descending run spells the natural 7th/6th with sharped letters instead of plain letters.

Standard Fux-tradition species-counterpoint pedagogy, directly relevant to Gradus's curriculum.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.