The verge and foliot escapement. The first mechanical clocks, building on the older principle of water flow for energy, employed a solid weight to yield the computation of time. If we just tied a weight and let it drop, we would not have to wait long, and we would be able to measure time only in seconds. So the trick for a mechanical clock without water, but instead using a weight, is to delay the time of the weight drop. This was done with an escapement. The verge is the vertical post and the foliot, the horizontally rotating bar. This animation (which can be viewed by clicking on the above image) is from Peter Ceperley who has a good explanation of the gear train powered via the escapement control. Think of this escapement is the medieval equivalent of a square wave--a mechanism to achieve an oscillating signal.