A Pendulum Clock is a mechanical clock that uses a pendulum to regulate the dividing of time. The use of the pendulum in a mechanical clock greatly improves the accuracey of the clock and was the first of several solutions used by mechanical clocks to regulate time dividing. The swinging of a pendulum has the important property of The time for a pendulum to swing back and forth is dependant only on the length of the pendulum, not the weight of the pendulum or the arc the pendulum swings through. This consistant mechanical action allows for the dividing of time to be regulated into the required time segments for proper use by the clock.

The original discovery of the properties of a pendulum, and an early design of a pendulum clock are accredited to Galileo. But the first constructed pendulum clock was the work of Christiaan Huygens in 1656. Huygens pendulum clock was accurate to one minute in a day, a big improvement over existing clocks that were only accurate to within fifteen minutes within a day. In 1721 George Graham improved the accuracey of the pendulum clock to within one second per day when he designed a pendulum clock that compensated for changes in pendulum length due to temperature.

Like all mechanical clocks the pendulum clock improved in accuracey over the years. The zenith of pendulum clock development came in 1921 with the W.H.Shortt clock. This pendulum clock has two pendulums, a master pendulum and a slave pendulum. The slave pendulum keeping the master pendulum in motion and also driving the clock hands, thus freeing the master pendulum from mechanical tasks that interefere with accuracey. This free pendulum clock achieved accuracey of 1/100th of a second in a day and was adopted by observatories for time keeping. The era of the pendulum clock was bought to a close with the arrival of the quartz crystal clock.