A penulum clock is a clock that uses a pendulum, a swinging weight, as its time base. From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world's most accurate 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.