A single note at 5 or more ledger lines fails legibility regardless of context or duration

Independent of run length or musical context, any single notehead requiring 5 or more ledger lines (ledgerLineCount(pitch, clef) >= 5) has crossed the point every cited authority treats as a legibility failure, not merely a stylistic preference. This is a harder, unconditional ceiling layered on top of ledger-line-sustained-threshold, which only fires on a RUN of 4+ notes — this rule fires on any single occurrence, including an isolated climactic note that ledger-line-sustained-threshold deliberately tolerates at 4 ledger lines.

A single note (a passing tone, a fermata peak, a chord extreme) at 5 or more ledger lines above or below the staff, engraved without a clef change or octave sign, regardless of how briefly it occurs or what surrounds it.

Read, Music Notation, states plainly that beyond about four ledger lines a note 'ceases to be quickly readable' at all — under Read's own wording, 5 ledger lines is already past that point. Gould, Behind Bars, gives no numeric ceiling but treats a 'rash of leger lines' as something to avoid outright, consistent with a hard ceiling existing somewhere below infinity. Stone, Music Notation in the 20th Century, notes that pre-20th-century engravings sometimes ran to 6+ ledger lines but that modern practice has moved decisively away from tolerating that. This chapter sets the hard ceiling at 5, not 4, specifically so a single expressive/climactic note AT 4 ledger lines — common and idiomatic, per ledger-line-sustained-threshold's own falsePositiveRisk note — is never itself flagged.

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