Sustained passage ledger-line ceiling before a clef change or octave sign is required

A SUSTAINED passage — four or more consecutive notes in one voice each requiring 3 or more ledger lines — should be re-clef'd or wrapped in an 8va/8vb bracket rather than engraved on ledger lines throughout. A single note or a brief 2-3-note excursion may go to 4 ledger lines without triggering a rewrite. (A single note at 5 or more ledger lines, regardless of run length, is a separate and harder failure — see ledger-line-absolute-ledger-ceiling.)

A cello passage sitting in bass clef with 4+ consecutive quarter notes at C5-E5 (4-5 ledger lines each) instead of switching to tenor or treble clef; a soprano line sustained above the treble staff at 3+ ledger lines for eight straight notes instead of an 8va bracket.

Gould, Behind Bars, cautions leger lines should be used 'as sparingly as possible' and explicitly prefers an octave line to 'a rash of leger lines'; she gives no single number. Ross, The Art of Music Engraving (Leger Lines chapter) is the most concrete of the four, describing 2-3 ledger lines as the normal working ceiling for continuous material and treating anything beyond as calling for 8va or a clef change. Read, Music Notation, states plainly that beyond about four ledger lines a note 'ceases to be quickly readable' at all. Stone, Music Notation in the 20th Century, notes that 20th-century practice increasingly favors octave signs over historical willingness to stack many ledger lines (she cites pre-20th-century engravings with 6+ ledger lines as a practice modern engraving abandoned). No authority gives one canonical number for the 'sustained' consecutive-note count — this is Gradus's own actionable threshold, chosen conservatively because the audience is a 15-year-old, transfer-blind, APD student for whom stacked ledger lines are exactly the kind of visual clutter the whole BSP/Gradus design philosophy exists to avoid.

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