had a lower coefficient of thermal expansion than metal. The rod is usually suspended from the clock frame with a short straight spring of metal ribbon (d); this avoids instabilities that were introduced by a conventional pivot. In the most accurate regulator clocks the pendulum is suspended by metal knife edges resting on flat agate plates. The pendulum is driven by an arm hanging behind it attached to the anchor piece (h) of the escapement, called the "crutch" (e), ending in a "fork" (f) which embraces the pendulum rod. Each swing of the pendulum releases the escape wheel, and a tooth of the wheel presses against one of the pallets, exerting a brief push through the crutch and fork on the pendulum rod to keep it swinging. Most quality clocks, including all grandfather clocks, have a "seconds pendulum", in which each swing of the pendulum takes one second (a complete cycle takes two seconds), which is approximately long from pivot to center of bob. Mantel clocks often have a half-second pendulum, which is approximately long. Only a few tower clocks use longer pendulums, the 1.5 second pendulum, long, or occasionally the two-second pendulum, which is used in the Great Clock of Westminster which houses Big Ben. The pendulum swings with a period that varies with the square root of its effective length. For small swings the period T, the time for one complete cycle (two swings), is where L is the length of the pendulum and g is the local acceleration of gravity. All pendulum clocks have a means of adjusting the rate. This is usually an adjustment nut (c) under the pendulum bob which moves the bob up or down on its rod. Moving the bob up reduces the length of the pendulum, reducing the pendulum's period so the clock gains time. In some pendulum clocks, fine adjustment is done with an auxiliary adjustment, which may be a small weight that is moved up or down the pendulum rod. In some master clocks and tower clocks, adjustment is accomplished by a small tray mounted on the rod where small weights are placed or removed to change the effective length, so the rate can be adjusted without stopping the clock. The period of a pendulum increases slightly with the width (amplitude) of its swing. The rate of error increases with amplitude, so when limited to small swings of a few