In multi-voice writing, each voice's clef changes independently — one voice changing clef never forces another to change

When two or more voices share a system (SATB grand staff, open-score fugue, stacked counterpoint), each voice's clef is chosen and changed entirely independently based on its OWN tessitura — a soprano voice moving into an unusually low passage does not drag the alto's clef along with it, and vice versa. The only shared-stave constraint is that voices sharing ONE physical stave (as in SATB's treble/bass pairing) necessarily share that stave's single clef; voices given their OWN stave (open score) are fully independent.

An open-score fugue render where a genuine per-voice clef choice (e.g. the alto-range middle voice choosing tenor clef for a sustained low passage) is prevented or overridden because the code couples all voices' clef choices together.

Gould, Behind Bars, 'Voices on One Staff' and Clefs chapters together establish this: independence is unlimited across separate staves, and necessarily shared only where two voices are forced onto one physical stave by the ensemble's layout convention (chorale grand staff). Ross, The Art of Music Engraving, agrees.

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