This "bass over the rib" caliper model is the first of its kind developed shortly after the M-BS clamp (simply a larger version of the "cello over the rib" caliper C-CAL). I simply added the best quality Koefer dial (Germany) to my M-BS clamp. Easily and rapidly measure the thickness of the Double Bass back with the top off (and the reverse also). Especially helpful when regraduating overly thick tops/backs.