Changing clef too often costs more than the ledger lines it saves

A clef change interrupts a reader's established pitch-reading frame and forces a brief re-orientation; authorities uniformly caution against changing clef merely because a passage's median pitch briefly crosses a threshold, if the passage then immediately crosses back. As a concrete guardrail: a clef change should not be introduced for an excursion that returns within roughly one bar (or, for the more forgiving octave-sign alternative, see octave-sign-preferred-over-ledger-threshold) — clef changes are for passages that GENUINELY settle into a new register for a meaningful span (conventionally several bars or more), not for every local peak or trough in the line.

A cello part that clef-changes to tenor for two notes, back to bass for the next two, back to tenor again, and so on for an entire passage whose overall tessitura is genuinely borderline — a textbook case for choosing ONE clef and tolerating some ledger lines (or a single octave sign) rather than clef-thrashing.

Gould, Behind Bars, Clefs chapter, states explicitly that frequent clef changes are worse than the ledger lines or octave signs they might avoid, and that a clef change should be reserved for a real, lasting change of tessitura. Ross, The Art of Music Engraving, agrees and adds the practical craft concern that each clef change also costs horizontal space (see clef-change-reduced-size below) which compounds the legibility cost of changing often. Read, Music Notation, agrees.

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