Voice crossing (simultaneous pitch-order inversion between adjacent voices) is normal in moderation and must not be flagged as an error on sight

Voice crossing occurs when two adjacent voices (e.g. Alto and Tenor) invert their expected pitch order WITHIN a single simultaneity — the lower-numbered voice sounds below the higher-numbered one at that instant. This is a real, accepted compositional device and is distinct from a shared-stave stem-role inversion, which is the engraving-visual symptom of the same event.

None in isolation — an isolated voice crossing is not itself a fault. Treating ANY crossing as a finding worth reporting on every occurrence is the false-positive this rule corrects (see shared-stave-role-inversion-threshold for the persistence-gated replacement).

General voice-leading literature (definition confirmed against standard part-writing references); Gould's shared-stave stem convention (voice-numbering-role-fixation) explicitly keeps the stem-role assignment fixed to the PART, not the momentary pitch — meaning a genuine voice crossing is expected to look visually 'inverted' on a shared stave and this is accepted engraving practice for an occasional crossing, not a defect.

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