A grandfather clock is a weight-driven mechanical timepiece encased in a tall, wooden cabinet that acts as an echo chamber for a cathedral chime melody. As the hand advances, the minute hand trips a star gear located on the central stem of the dial face. The point of the start gear lifts a pin that triggers the turning of the music roll (just as you would find in a music box).
As the music roll turns, it pulls back a chime hammer, which then falls back to its original position and strikes a chime rod. Chime rods are cut at various lengths to produce different notes. These hammers and rods, struck in various orders, will produce the selected cathedral chime melody - Westminster, St. Michaels or Whittington.
The moon dial tracks the 29½ day Lunar Cycle. A separate gear on the centre shaft of the dial, trips a pin once every 24 hours which advances the dial one "click" per one day. The phases of the moon (i.e. new, quarter, half, full, etc.) are depicted through the Eastern and Western Hemispheres (1/2 circles located on each side of the moon dial) bisecting the moon through its rotation.
Pendulums are a vital element of a grandfather clock. The swing of the pendulum (the amount of time it takes the pendulum to go back and forth once) will remain constant while the weights in a clock fall. Variances in time-keeping are caused not by the weight of the pendulum but the length.
Most quality grandfather clocks feature a time adjustment nut at the bottom of the pendulum to adjust the length of the pendulum. Winding this nut up or down the thread will increase or decrease the length of the pendulum which in turn will slow down or speed up the clock.