The pendulum clock revolutionized timekeeping in Western Europe by making it much more precise: Instead of accuracy within about fifteen minutes per day, clocks became accurate within about fifteen seconds per day. Although the rudimentary idea of the pendulum clock may be credited to Galileo as early as 1637, it was Christiaan Huygens, working independently, who developed the functional models that would change the way clocks were generally made.

Since the invention of the mechanical clock in the thirteenth century, before Huygens all clocks used a verge escapement to regulate their rate. Pallets attached to one end of the verge were pushed by the teeth of the last wheel of the going train. The other end of the verge was attached to a balance wheel, or foliot. The action of the teeth and pallets rotated the balance wheel in one direction and then in the other, and this motion slowed down and regulated the going train. However, verge escapements were not very accurate because the oscillation period was faster with heavier weights or a tighter spring.

Huygens's great discovery was to use the pendulum as a regulating device. Instead of pallets attached to the verge, the pendulum's shaft regularly hit a crutch attached to the balance wheel, thereby forcing the clock's going train to follow its beat. In order to make this possible, Huygens turned the clock's mechanism 90œ so that the balance wheel oscillated vertically. The pendulum itself was repeatedly nudged into motion by the force exerted by weights on the clock's going train, its swing being regulated by a small movable weight. The pendulum shaft was attached to the back of the clock to keep the pendulum as free as possible from interference. As the period of a pendulum swinging freely according to a circular path varies according to the magnitude of its swing, the arc of the pendulum itself was guided by curved metal "cheeks" placed on either side of the suspension cord, effectively shortening the length of that cord and causing its bob to swing according a noncircular, cycloidal motion. As the pendulum swung upward around the cheeks, it swung faster--just the right amount faster so that if one began the pendulum's swing at any point on the curve, the force of gravity would carry it to the bottom at the same time, a property called isochronism. Huygens had quickly understood that circular pendulums were not isochronous unless they were of very small amplitude so that the difference in curvature was negligible. Later pendulum clocks would be refined by anchor and other sorts of escapements that removed the need for the cycloidal cheeks by sufficiently limiting the pendulum's arc.

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