A beam must never cross a notated barline

Beamed groups are always contained within a single measure; a beam that visually spans a barline is a fundamental engraving error in common-practice notation.

Gould, Behind Bars, ch. Beaming — states this as an absolute rule for standard notation; Ross, Art of Music Engraving — same; Stone, Music Notation in the 20th Century, notes some 20th-century avant-garde scores deliberately beam across barlines to show a phrase overriding the notated meter, framed explicitly as an exceptional, modern device, not standard practice.

Recommendation for Gradus: treat as an unconditional error — the curriculum's scope (Fux species counterpoint, Bach chorale, fugue, common-practice harmony) never legitimately needs Stone's avant-garde exception, so no opt-out should exist.

Our own judgement, not a sourced convention — shown separately so the two are never confused.

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