Chromatic passing tones: ascending motion spells with sharps, descending motion spells with flats

A chromatic passing tone between two diatonic degrees a whole step apart is spelled as the sharped lower neighbor ascending, or the flatted upper neighbor descending (C4-C#4-D4 ascending; D4-Db4-C4 descending).

An ascending passing tone between C4 and D4 is spelled Db4 instead of C#4; a descending one between D4 and C4 is spelled C#4 instead of Db4.

Standard voice-leading pedagogy relevant to Gradus's chorale-harmony curriculum; not itself an engraving-manual DISPLAY rule (Gould/Read/Ross adjudicate glyph display, not melodic spelling choice) but the specific SPELLING convention the commissioning brief names explicitly.

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