A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator, meaning it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates.
From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world’s most precise timekeeper, accounting for its widespread use.
Pendulum clocks must be stationary to operate; any motion or accelerations will affect the motion of the pendulum, causing inaccuracies, so other mechanisms must be used in portable timepieces. They are now kept mostly for their decorative and antique value.