External versus internal gearsExternal gears are gears with teeth formed on the outer surface of a cylinder or cone. In contrast, an internal gear is a gear with teeth formed on the inner surface of a cylinder or cone. As with bevel gears, internal gears are gears with pitch angles greater than 90 d

The development of gear measurement technology​ has a history of nearly a hundred years. Corresponding to gear accuracy standards, modern gear measurement techniques can be classified into the following three types.