An annular gear having teeh on the inner surface of its rm .The internal gear always meshes with an external gears .

When meshing two external gears ,he rotation occurs in the oppositive directions.When meshing an internal gear with an external gear the rotation occus in the same direction .

Care should be taken with regar to the number of teeth on each gear when meshing a large (internal ) gear with a small (external) gear ,since three types of interference can occur .

Usually internal gears a driven by small external gears .

Allows for a compact design of the machine .