The largest and most complex of all musical instruments, the conventional, or pipe, organ produces its sound when air, actuated by a keyboard, is blown through pipes of graduated sizes. (Electric and electronic organs substitute electromechanical or electronic devices to produce an organ sound, and may surpass the pipe organ in the variety of sound, and may surpass the pipe organ in the variety of sounds they can create.)
The pipes may be of the flue type, in which the air column inside the pipe is set into vibration by a stream of air passing over a sharp edge at the base of the pipe; or they may be of the reed type, activated by a beating reed, consisting of a slightly curved metal tongue vibrating in a slit in a metal tube (the shallot) set into the base of the pipe. The pipes are made in various cylindrical and conical shapes to produce distinctive timbers, or tone colors. Each rank, or set of pipes, consist of a series of pipes of one tonal design, gradated in size to produce all the pitches within the instrument's compass. In addition to sets of pipes at normal or "8-foot" pitch (so named because the pipe for the lowest note in such a rank, C is approximately 8 feet long), there are sets of pipes constructed at 16-foot, 4-foot, 2-foot, and 1-foot pitches that sound, respectively, an octave below, or one, two, or some ranks built at pitches other than the octave, for example, the quaint (5 1/3 feet), the twelfth (2 2/3 feet), the tierce (1 3/5 feet).
The feet of the pipes are set into a channeled box, the "wind chest." A system of valve in the wind chest (the pallets), controlled through the action by keyboard, admit air from the wind supply to sound the pipes. The wind supply may consist of manually or mechanically driven bellows or an electric blower. Linkage between the valves in the wind chest and the keyboard may be mechanical, peumatic, electrical, or a combination thereof.
The organ is played by one or more keyboards, each of which usually controls one division of pipe work, or more than one division through coupling, located on its own wind chest. In addition to manuals (keyboards played by the fingers of the hands) an organ usually has a keyboard for feet, or pedal board. A thin batten called a slider passes under each rank of pipes. Controlled by stop knob located near keyboards, sliders govern the number of ranks, or stop, in use at any one time.