When two parallel and coplaner shafts are connected by gears having teeth parallel to the axis of the shaft called spur gears, the arrangement is known as spur gearing.

Another name given to the spur gearing is helical gearing in which the teeth are inclined to the axis of the shaft. There are single and double helical gears. The double helical gears are known as herringbone gears.

A worm drive is a gear arrangement in which a worm meshes with a worm gear The two elements are also called the worm screw and worm wheel.

Pitch circle is the imaginary circle on the gear about which it may be supposed to roll without slipping with pitch circle of another gear. The point of contact of two pitch circle becomes the pitch point. Look out the two dotted circles which are in pure rolling about a fixed point ( which is called the pitch point).

In a gear, the pitch diameter divided by the number of teeth is called as Module.