DP is shorthand for Diametrical Pitch and is the number of teeth that will fit on 1" of the circumference of the PCD or Pitch Circle Diameter.

Two main diameters on a gear that the user needs to know, many others but you can forget them.

OD, Outside diameter is obvious and the PCD is a theoretical diameter inside the PCD that if the gear had no teeth and transmitted drive by friction [ 2 rubber rollers ] would be the PCD.

Basically a point on the PCD that contact the other gear.

DP is found by adding 2 teeth to the number of teeth and dividing it by the OD.

So a gear 1.100" OD with 20 teeth = 20 +2 =22 / 1.1 =20 DP

The PCD is number of teeth divided by the DP so 20 / 20 = 1.00"

If you have two gears identical to above the centres will be 1/2 PCD + 1/2PCD so 1.00" in this case.

.