In music, the organ is a keyboard instrument of one or more pipe divisions, each played with its own keyboard, played either with the hands on a keyboard or with the feet using pedals. The organ is a relatively old musical instrument, dating from the time of Ctesibius of Alexandria, who invented the water organ. It was played throughout the Ancient Greek and Ancient Roman world, particularly during races and games. During the early medieval period it spread from the Byzantine Empire, where it continued to be used in secular (non-religious) and imperial court music, to Western Europe, where it gradually assumed a prominent place in the liturgy of the Catholic Church. Subsequently it re-emerged as a secular and recital instrument in the Classical music tradition.

Most organs in Europe, the Americas, and Australasia can be found in Christian churches. The introduction of church organs is traditionally attributed to Pope Vitalian in the 7th century. Due to its simultaneous ability to provide a musical foundation below the vocal register, support in the vocal register, and increased brightness above the vocal register, the organ is ideally suited to accompany human voices, whether a congregation, a choir, or a cantor or soloist. Most services also include solo organ repertoire for independent performance rather than by way of accompaniment, often as a prelude at the beginning the service and a postlude at the conclusion of the service. Organs are also used to give recital concerts, called organ recitals. In the early 20th century, symphonic organsflourished in secular venues in the United States and the United Kingdom, designed to replace symphony orchestras by playing transcriptions of orchestral pieces. Symphonic and orchestral organs largely fell out of favor as the orgelbewegung (organ reform movement) took hold in the middle of the 20th century, and organ builders began to look to historical models for inspiration in constructing new instruments. Today, modern builders construct organs in a variety of styles for both secular and sacred applications.

The pump organ, or harmonium, was the other main type of organ before the development of electronic organs. It generated its sounds using reeds similar to those of a piano accordion. Smaller, cheaper and more portable than the corresponding pipe instrument, these were widely used in smaller churches and in private homes, but their volume and tonal range was extremely limited, and they were generally limited to one or two manuals, pedalboards being extremely rare. The chord organ was invented by Laurens Hammond in 1950.[9] It provided chord buttons for the left hand, similar to an accordion. Other reed organ manufacturers have also produced chord organs, most notably Magnus from 1958 to the late 1970s.[10]

The organ has had an important place in classical music, particularly since 1500. Spain’s Antonio de Cabezón, the Netherlands’ Jan Pieterszoon Sweelinck, and Italy’s Girolamo Frescobaldi were three of the most important organist-composers before 1650. Influenced in part by the latter two of these men (not by Cabezón), the North German school rose from the mid-17th century onwards to great prominence, with leading members of this school having included Buxtehude, Franz Tunder, Georg Böhm, and above all Johann Sebastian Bach, whose contributions to organ music continue to reign supreme. During this time, the French Classical school also flourished. François Couperin, Nicolas Lebègue, André Raison, and Nicolas de Grigny were French organist-composers of the period. Bach knew Grigny’s organ output well, and admired it. In England, Handel was famous for his organ-playing no less than for his composing; several of his organ concertos, intended for his own use, are still frequently performed.

After Bach’s death in 1750, the organ’s prominence gradually shrank, as the instrument itself increasingly lost ground to the piano. Nevertheless, Felix Mendelssohn, César Franck, and the less famous A.P.F. Boëly (all of whom were themselves expert organists) led, independently of one another, a resurgence of valuable organ writing during the 19th century. This resurgence, much of it informed by Bach’s example, achieved particularly impressive things in France (even though Franck himself was of Belgian birth). Major names in French Romantic organ composition are Charles-Marie Widor, Louis Vierne, Alexandre Guilmant, Charles Tournemire, and Eugène Gigout. Of these, Vierne and Tournemire were Franck pupils.