Voice crossing — precise definition and generalized detection

Voice crossing occurs when, AT THE SAME MOMENT (same onset), a nominally LOWER voice sounds a pitch HIGHER than a nominally higher voice (or vice versa). Generally avoided in strict four-part writing (obscures which voice is which) though occasionally used deliberately, in which case it should be a marked exception, not an accident.

At beat 2, Alto sounds A4 (69) while Soprano sounds F4 (65) at the same beat.

Standard part-writing pedagogy (Kostka & Payne et al.); the engraving-side consequence is discussed implicitly by Gould's stem-opposition convention (score-chorale-two-stave-stems-opposed), since a crossed note's forced stem now points the 'wrong' way relative to its actual pitch.

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