Compound-meter beat grouping (6/8, 9/8, 12/8, 6/16, 12/16)

In compound time, the beat unit is a dotted note; beams group in threes of the written unit (three eighths per beam in 6/8, 9/8, 12/8 — three sixteenths per beam in 6/16, 12/16), yielding 2, 3, or 4 groups per bar respectively.

6/8 content beamed in two groups of three sixteenths incorrectly split as 2+2+2 eighths (duple grouping) rather than 3+3.

Gould, Behind Bars, ch. Beaming — compound-time grouping in threes; Ross, Art of Music Engraving — same convention as the historical norm for 6/8-family meters.

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