Gear-train is a series of different wheels, each equipped with a wheel and a pinion on the same axis. Thus the rotation can be multiplied or divided. For example, in a mechanical watch, when a barrel makes one revolution in 6 hours, the escape wheel, which is located four mobiles further on, will make 8,640 revolutions.

Conversely, when an anchor wheel makes one revolution, the barrel will move by a theoretical angle of 0.0417 degrees. Also it is important to note that with each mobile the direction of rotation is reversed.