A grandfather clock has a clock face, pendulum and weights enclosed in a thin, tall freestanding wooden case. The grandfather clock has also been known by the names longcase clock, tall-case clock and floor clock. William Clement, an English clockmaker developed the longcase clock form in 1670. The name grandfather clock for a tall-case clock is thought to have come from the 1875 song, My Grandfather's Clock, by Henry Clay Work.

The typical grandfather clock is about 6 feet (1.8 m) to 8 feet (2.4 m) high. The wooden part of the clock that encases the clock face is called the bonnet, or hood. Often, the bonnet of a grandfather clock has elaborate carvings such as spindles. Wood types used for grandfather clocks vary, but oak and mahogany are the most popular woods used to make antique grandfather clocks.

An antique grandfather clock usually has a painted or brass dial. Some antique grandfather clocks have a moon or rocket ship motif. A grandfather clock is a striking clock as it strikes the time each hour or portions of each hour. Chimes ring to announce the time. Only a few antique grandfather clocks were made with musical bells or reeds to give them an organ-like sound.

A grandfather clock may have a moon phase dial marked by two moons painted on the dial. A moon phase dial works the same as clock hands. The minute hand moves one revolution per hour and the hour hand moves one revolution every 12 hours. The moon phase dial moves at one revolution about every 56 days as the moon's cycle is 28 days.

An antique grandfather clock usually has either an eight day or a 30 hour movement. Most grandfather clocks with an eight day movement are wound with a key and have two winding holes. The weight that makes the clock strike is located on the left of the clock's front, while the weight that keeps the clock moving and telling time is on the right of the clock's front. The weights in a grandfather clock also give the energy for the clock hands to turn.

The 30 hour movement on some grandfather clocks is wound by a chain or a rope and not a key as with an eight day movement. The 30 hour movement has just the weight that both runs the grandfather clock and causes the clock to strike. Usually, the 30 day variety of grandfather clock costs less than the 8 day type.

Pendulum clocks such as the grandfather clock were first used in 1656 and were the first clocks to be accurate. The pendulum helps keep clocks running accurately. Christiaan Huygens, a Dutch astronomer, suggested in the 1600s that a pendulum could be used to create a more accurate clock. A pendulum in a grandfather clock, especially a very large one, may swing once every two seconds, while the pendulum on a wall clock may swing every second and the pendulum on a cuckoo clock may swing twice each second.