A gear system enables the transmission of motion. In this example, which deals exclusively  with toothed wheels, only rotational motion is dealt with.  The speeds of rotation are a function of the gear ratio (or transmission ratio), which depends solely on the number of teeth on each wheel.

Knowing that the number of teeth is proportional to the wheel's diameter, we can also define the gear ratio as a function of wheel diameters.

A gear system ensures  a transmission without slippage, and with a very high yield. The gear ratio can be greater than 1 (muliplication) or less than 1 (reduction).  Augmentation of the couple  produces a decreased speed, and vice versa.