The pendulum, according to Webster's Third International Dictionary, is: (a) "a body suspended from a fixed point so as to swing to and fro under the action of gravity and commonly used to regulate the movements of clockwork and other machinery;" or (b) "a suspended body that vibrates not by swinging but by rotating, with alternate twisting and untwisting (as the balance wheel of a watch) — called also torsion pendulum." In the case of the swinging pendulum, the period is constant and more or less independent of the amplitude and of the swing, angular velocity of the swing, and of the mass of the bobbin. Because the pendulum swings with a regular motion of equal periods without regard to amplitude or mass, it is useful as a timekeeper.