organ, in music, a keyboard instrument, operated by the player’s hands and feet, in which pressurized air produces notes through a series of pipes organized in scalelike rows. The term organ encompasses reed organs and electronic organs but, unless otherwise specified, is usually understood to refer to pipe organs. Although it is one of the most complex of all musical instruments, the organ has the longest and most involved history and the largest and oldest extant repertoire of any instrument in Western music.

In spite of far-ranging technical developments, the organ’s basic principles of operation remain substantially unchanged from when they were discovered more than 2,000 years ago. Conventional pipe organs consist of four main parts: a keyboard or keyboards and other controls, pipes to produce the tone, a device to supply wind under pressure, and a mechanism connected to the keys for admitting wind to the pipes. The most basic instrument consists of a single set, or rank, of pipes with each pipe corresponding to one key on the keyboard, or manual. Organs usually possess several sets of pipes (also known as stops, or registers), however, playable from several keyboards and a pedal board. Under their control are the various ranks of wooden and metal pipes of differing length and shape. These fall into the two distinct categories of flue pipes and reeds.