Ledger-line length, thickness, and spacing relative to the staff

A ledger line is drawn only as long as necessary to pass through (or just past) its notehead — conventionally extending about a quarter of a notehead's width beyond the notehead on each side, never a full staff-width line. Its thickness matches the ordinary staff-line weight (a hairline), NOT the heavier barline weight. Vertical spacing between successive ledger lines, and between the staff's own edge line and the first ledger line, equals the staff's own line-to-line distance exactly — ledger lines are a literal continuation of the staff grid, not an independently-spaced element.

A ledger line rendered full stave-width (as if it were a barline or a stave-extension), or one noticeably thicker than the staff lines it continues, or ledger-line spacing that visually diverges from the staff's own line spacing (e.g. compressed to save vertical room), all read as either a barline mistake or a broken staff to an experienced reader.

Ross, The Art of Music Engraving, Leger Lines section, gives the quarter-notehead-width overhang explicitly as the traditional engraver's measurement. Gould, Behind Bars, states the equal-spacing-continuation rule. SMuFL's engraving defaults (`legerLineThickness`, `legerLineExtension` metadata keys) codify both numbers for font-based renderers, and Dorico/LilyPond both default to extending a ledger line a small fixed fraction of a notehead-width past each side.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.