Spacing between adjacent upper voices

In four-part writing, the interval between Soprano and Alto, and between Alto and Tenor, should not exceed an octave (12 semitones) at any simultaneity. The Tenor-Bass interval is NOT subject to this restriction and may exceed an octave freely, since the bass frequently leaps to outline root-position harmony with no upper-voice register above it to collide with.

Soprano at C5 (72) with Alto at D3 (50) — a 22-semitone gap, far exceeding the 12-semitone S-A ceiling.

Kostka & Payne / Aldwell & Schachter (standard part-writing spacing rule) — harmony-pedagogy authority, not a pure engraving-manual rule, but load-bearing for Gradus chorale content.

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