A pair of mating gears can be viewed as a cam and follower mechanism designed to use the rotary movement of an input shaft to drive the rotary movement of an output shaft. This is achieved by providing a series of cam and followers, or gear teeth, distributed around the circumferences of two circles that form the mating gears. Early implementation of this rotary movement used cylindrical and rectangular teeth without concern for smooth transmission of movement, while the teeth were engaged---see the photo of the main drive gears for the windmill Doesburgermolen in Ede, Netherlands.