When an 8va/8vb/15ma/15mb bracket is preferable to continued ledger lines

Once a passage crosses the sustained-ledger threshold (ledger-line-sustained-threshold: 4+ consecutive notes at 3+ ledger lines), the preferred fix is an 8va (for above-staff excursions) or 8vb (for below-staff excursions) bracket over the passage, NOT a clef change, when the excursion is temporary (expected to return to the normal register within the same system or the next one or two). A clef change is preferred instead when the excursion is genuinely sustained across a longer span (see clef-choice-minimize-across-passage below) — octave signs are for a temporary register excursion inside music that otherwise belongs on the current clef; clef changes are for a genuine, lasting shift of tessitura.

A four-bar violin passage sitting a tenth above the staff, engraved throughout on 4-5 ledger lines rather than under a single 8va bracket, or conversely a lasting change of register (e.g. an instrumental part that has simply moved to a new comfortable range for the rest of the piece) forced under an 8va bracket rather than given a proper clef change.

Gould, Behind Bars (Octave signs chapter) states this preference explicitly: 'a passage that goes temporarily very high or low is usually better treated with an octave line than either a rash of leger lines or a clef change and change back.' Ross, The Art of Music Engraving, agrees. Read, Music Notation, frames the choice as a cost-benefit between the reader's need to track two temporary changes (into the octave sign and back out) versus a run of ledger lines, generally favoring the octave sign for anything under roughly a system's worth of music. 15ma/15mb (two octaves) is reserved by all four authorities for extreme piano writing and is essentially never appropriate for the vocal/chamber-instrument tessituras Gradus teaches.

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