A mechanical wind-up metronome in motion
One common type of metronome is the wind-up metronome, which uses a weight on the end of a rod to control the tempo. The rod swings back and forth in tempo; mechanics inside the metronome produce a clicking sound on each swing of the rod.
Most newer metronomes are electronic. The simplest electronic metronomes have a dial or buttons to control the tempo; some can also produce a tuning note (usually A, 440 hertz). The button forms range from simple credit-card sized devices to the complicated "Dr. Beat", manufactured by Boss. In addition to a simple pulse, this metronome can play polyrhythms and can "count aloud", using a sampled voice.