A tall mechanical clock, usually between six and seven and a half feet tall and built with a carved wooden frame and a detailed clock face at the top. Grandfather clocks are powered by a long pendulum and one or more weights on long chains, which take up most of the vertical space inside the clock. The weights serve the same purpose as the winding knob on a mechanical wristwatch, tightening the spring and providing resistance so that its constant tension can drive the timekeeping gears.
The long shape and wooden frame led to the rather morbid name "coffin clock" when these were first produced. The name that's currently used comes directly from the 1876 song "My Grandfather's Clock" by Henry Clay Work.