The bolt circle diameter (BCD) is the diameter of the circle made by the chainring bolts. On four arm cranks its the distance between to chainring bolts. There are two BCD measurements for a triple chainset - The middle & outer share one BCD and the inner has another. 4 ARM Cranksets have a 104mm middle and outer BCD and 64mm inner BCD. 5 ARM Compact drive have a 94mm middle and outer BCD and 58mm inner BCD. 5 ARM (older) Cranksets have a 110mm middle and outer BCD and 74mm inner BCD.