Voice overlap — precise definition, distinct from crossing

Voice overlap is a TEMPORAL (not simultaneous) fault: a voice moves to a new pitch that is beyond the pitch its adjacent neighbor voice held immediately before — even though the two voices are never simultaneously crossed at any single instant. This is distinct from and commonly missed alongside simultaneous crossing, because two adjacent voices can pass through each other's registers across time without their pitches ever coinciding at one shared beat.

Soprano holds E5 (76) as a whole note from beat 0. Alto, moving in quarter notes underneath, reaches F5 (77) at beat 3 — Alto's new pitch (77) exceeds Soprano's most recent onset (76), even though the two voices never share a simultaneous onset at beat 3 for the ordinary crossing check to catch (Soprano's next onset is at beat 4). This is a pure overlap: no simultaneous crossing fires, but Alto has moved into registral territory Soprano is still occupying.

Aldwell & Schachter (Harmony and Voice Leading) gives this exact definition and explicitly distinguishes it from simultaneous crossing.

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