A musical keyboard is the set of adjacent depressible levers or keys on a musical instrument. Keyboards typically contain keys for playing the twelve notes of the Western musical scale, with a combination of larger, longer keys and smaller, shorter keys that repeats at the interval of an octave. Depressing a key on the keyboard causes the instrument to produce sounds, either by causing a hammer to mechanically strike a string or tine (piano, electric piano, clavichord); pluck a string (harpsichord); causing air to flow through a pipe (pipe organ or accordion); or strike a bell (carillon in a church tower). On electric and electronic keyboards, depressing a key connects one or several circuits (Hammond organ, digital piano, synthesizer, MIDI controller keyboard) which causes sounds to be produced. A pedal keyboard is a keyboard used with a pipe organ or theatre organ which is played with the feet. Since the most commonly encountered keyboard instrument is the piano, the keyboard layout is often referred to as the "piano keyboard".