It happens to the best of ensembles: In the middle of rehearsal, something will go wrong. Rather than start all the way back at the beginning of the piece — which would waste precious rehearsal time, not to mention result in rehearsing things that are already solid — the group will pick things up somewhere in the middle. To get everyone on the same page, they’ll often find a rehearsal mark.

Rehearsal marks — in this format specifically, they’re rehearsal letters — and composers (or editors or engravers) will dot them thruout the score to mark significant landmarks in the piece to aid in the rehearsal process. In traditional Western musical notation, and still as the default setting for most engraving software today, they’ve taken the form pictured above: A capital letter inside a circle or a box. This is an unfortunate practice that should absolutely be discontinued — basically all rehearsal marks should be boxed bar numbers instead.