Early linear motion bearings consisted of a row of wooden poles placed under a row of pry plates. Modern linear bearings work on the same principle, but sometimes balls are used instead of rollers. The simplest rotary bearing is the bush bearing, which is simply a bushing between the wheel and the axle. This design was then replaced by rolling bearings, in which the original bushing was replaced by a number of cylindrical rollers, each of which acted like a separate wheel.