forward it also gives the pendulum or balance wheel a push to keep it moving. Oscillator The timekeeping element, either a pendulum or a balance wheel. It swings back and forth, with a precisely constant time interval between each swing, called the beat. A pendulum movement has a pendulum hangar usually attached to a sturdy support on the back, from which the pendulum is suspended and a fork which gives the pendulum impulses. The oscillator always has some means for adjusting the rate of the clock. Pendulums usually have an adjustment nut under the bob, while balance wheels have a regulator lever on the balance spring. Types of movements Watch movements come in various shapes to fit different case styles, such as round, tonneau, rectangular, rectangular with cut corners, oval and baguette, and are measured in lignes, or in millimetres. Each specific watch movement is called a caliber. The movement parts are separated into two main categories: those belonging to the ébauches and those belonging to the assortments. In watch movements the wheels and other moving parts are mounted between two plates, which are held a small distance apart with pillars to make a rigid framework for the movement. One of these plates, the front plate just behind the face, is always circular, or the same shape and dimensions as the movement. The back plate has various shapes: Full plate movement In this design, used in the earliest pocketwatches until the 18th century, the back plate was also circular. All the parts of the watch were mounted between the two plates except the balance wheel, which was mounted on the outside of the back plate, held by a bracket called the balance cock. Three-quarter plate movement In the 18th century, to make movements thinner, part of the back plate was cut away to make room for the balance and balance cock. Bridge movement In modern watch movements, the back plate is replaced with a series of plates or bars, called bridges. This makes servicing the movement easier, since individual bridges and the wheels they support can be removed and installed without disturbing the rest of the movement. The first bridge movements, in Swiss pocketwatches from around 1900, had three parallel bar bridges to support the three wheels of the going train. This style is called a three finger or Geneva movement. Mechanical watch movements are also classified as manual or