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.

The pipes are arranged over a wind chest that is connected to the keys via a set of pallets, or valves, and fed with a supply of air by electrically or mechanically activated bellows. Each rank is brought into action by a stop that is connected by levers, or electrically, to a slider. To bring a pipe into speech the player must first draw a stop to bring the holes in the slider into alignment with the foot of the pipes on the toe board. Pressing a key causes the pallet under that pipe to open, allowing air to travel along a narrow channel, through the slider hole, and into the pipe.

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Mechanical action, until the 19th century the sole method of linkage between pipe and keyboard, is still commonplace. The usual forms of mechanically aided action are tubular pneumatic, electro-pneumatic, and direct electric. An organ is generally disposed in divisions, each with a number of separate ranks controlled by separate manuals (two manuals and pedals are the minimum required to play the bulk of the legitimate repertoire). A large instrument may have five, or exceptionally more, manuals banked above each other, each controlling families of tones and pitches.

A characteristic of the organ is the freedom it allows the player to build up the volume and timbre by adding, to the basic tone, stops of proportionately higher or lower pitch. The pitch of any pipe is proportional to its length. Thus, an 8-foot (2.4-metre) pipe will sound at normal keyboard pitch, one of 16 feet (5 metres) will sound its sub-octave, and one of 4 feet (1 metre), its octave. Mutation stops sound at pitches corresponding to the harmonics of unison pitch. Pipes may vary from 32 feet (10 metres) long to less than 1 inch (2.5 cm), giving the organ a possible range of nine octaves—larger than any other instrument.

The earliest known organ was the hydraulis of the 3rd century bce, a rudimentary Greek invention, with the wind regulated by water pressure. The first recorded appearance of an exclusively bellow-fed organ, however, was not until almost 400 years later. By the 8th century organs were being built in Europe, and from the 10th century their association with the church had been established. The 15th and 16th centuries witnessed significant tonal and mechanical advances and the emergence of national schools of organ building. By the early 17th century all the essential elements of the instrument had been developed, and subsequent developments involved either tonal changes or technological refinements.

It was during the High Baroque period that the organ reached its greatest popularity and found its most important composer in Johann Sebastian Bach (1685–1750). There existed at this time two principal schools of organ building: the French, with its colourful reeds and mutations, and the German and Dutch, with their outstanding choruses.

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After the death of Bach, organ building entered into a gradual decline, most particularly in Germany and England, where organs built after 1800 were of increasingly poor tonal quality. There was, however, more emphasis on orchestrally imitative stops. The 19th century also saw the widespread introduction of reed organs, such as the harmonium and the melodeon. Reed organs produce sound by using freely vibrating reeds (rather than the beating reeds used in the reed pipes of pipe organs), usually without resonators. Smaller and less complicated than pipe organs, they remained popular in homes and small institutions until the early 20th century, when they lost ground to electronic organs and mass-produced pianos.

The 20th century witnessed both the revival of classical ideals in organ construction and the reemergence of the organ as an independent instrument commanding its own idiomatic literature. When Laurens Hammond introduced the electronic organ in the United States in 1935, it provided an economical and compact substitute for the organ, but its imitative sounds have never been able to reproduce the tonality of the pipe organ.