In order for two gears to be compatible, the must be the same type ( spur, helical, etc.), have the same pressure angle, and the diametral pitch (Imperial) or module (metric). You can't mix metric and Imperial gears. There are some pitches of metric gears which are close to that of Imperial gears and appear to mesh but actually don't. An Imperial gear with a diametral pitch of 16 is close to a mod 1.5 metric gear, for example. The mod 1.5 metric gear, in Imperial measurement, will have a diametral pitch of 16.9. A mismatched set would wear quickly.

This can be a problem when trying to identify the gear pitch or modulus as the pitch diameter is what must be measured. Diametral pitch is the ratio of the number of teeth to the pitch diameter in inches and module is the ratio of the pitch diameter in mm to the number of teeth.

You can closely approximate the pitch diameter by measuring the distance from the root of one tooth to the O.D. on the opposite side. Another method is to measure the O.D. and divide into or by the number of teeth +2 for Imperial or metric respectively.