Cogged Belt

A COG belt is a belt with teeth. That is instead of a smooth belt, such as a simple v-belt, that will always have some slip, and whose drive ratio is then only a close approximate ratio, a “COGGED” belt has teeth, so that the drive ratio is an exact value based on the number of teeth in each pulley (or sprocket) that the belt passes over.

Toothed belts also permit much higher torque to be driven than not toothed belts.

So, the COG number of a pulley is the number of teeth in the pulley.