Chromatic button accordion layouts: C versus B system

Chromatic button accordions like my new French one come in two main keyboard arrangements: C system and B system. Pics below, C on the left. The hand is at the right side as you play. Martin was curious about the difference, and I explained it as diagonals of rising musical notes going one way on the C system, and on the other diagonal on B. Which wasn’t easy for him to visualise! So I just screen shotted portions of the two layouts. You can get up to 5 rows of treble buttons on a chromatic button accordion, but 3 are essential, and show the key layout differences. I have 4 rows on mine, which adds handy extra fingering options.