The following four systems of gear teeth are commonly used in practice:

1.   14 ½^O  Composite system

2.   14 ½^O  Full depth involute system

3.   20^O  Full depth involute system

4.   20^O Stub involute system

The 14½^O composite system is used for general purpose gears.

It is stronger but has no interchangeability. The tooth profile of this system has cycloidal curves at the top and bottom and involute curve at the middle portion.

The teeth are produced by formed milling cutters or hobs.

The tooth profile of the 14½^O full depth involute system was developed using gear hobs for spur and helical gears.

The tooth profile of the 20^o full depth involute system may be cut by hobs.

The increase of the pressure angle from 14½^o to 20^o results in a stronger tooth, because the tooth acting as a beam is wider at the base.

The 20^o stub involute system has a strong tooth to take heavy loads.