The pipe organ is a musical instrument that produces sound by driving pressurized air (called wind) through the organ pipes selected from a keyboard. Because each pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass. Most organs have many ranks of pipes of differing timbre, pitch, and volume that the player can employ singly or in combination through the use of controls called stops.

A pipe organ has one or more keyboards (called manuals) played by the hands, and a pedal clavier played by the feet; each keyboard controls its own division, or group of stops. The keyboard(s), pedalboard, and stops are housed in the organ's console. The organ's continuous supply of wind allows it to sustain notes for as long as the corresponding keys are pressed, unlike the piano and harpsichord whose sound begins to dissipate immediately after a key is depressed. The smallest portable pipe organs may have only one or two dozen pipes and one manual; the largest may have over 33,000 pipes and seven manuals.^[2] A list of some of the most notable and largest pipe organs in the world can be viewed at List of pipe organs. A list consisting the ranking of the largest organs in the world—based on the criterion constructed by Michał Szostak, i.e. 'the number of ranks and additional equipment managed from a single console—can be found in 'The Organ'^[3] and in 'The Vox Humana'.^[4]

The origins of the pipe organ can be traced back to the hydraulis in Ancient Greece, in the 3rd century BC,^[5] in which the wind supply was created by the weight of displaced water in an airtight container. By the 6th or 7th century AD, bellows were used to supply Byzantine organs with wind.^[5][6] Beginning in the 12th century, the organ began to evolve into a complex instrument capable of producing different timbres. A pipe organ with "great leaden pipes" was sent to the West by the Byzantine emperor Constantine V as a gift to Pepin the Short, King of the Franks, in 757.^[7] Pepin's son Charlemagne requested a similar organ for his chapel in Aachen in 812, beginning the pipe organ's establishment in Western European church music.^[8] In England, "The first organ of which any detailed record exists was built in Winchester Cathedral in the 10th century. It was a huge machine with 400 pipes, which needed two men to play it and 70 men to blow it, and its sound could be heard throughout the city."^[9] By the 17th century, most of the sounds available on the modern classical organ had been developed.^[10] From that time, the pipe organ was the most complex man-made device^[11]—a distinction it retained until it was displaced by the telephone exchange in the late 19th century.^[12]

Pipe organs are installed in churches, synagogues, concert halls, schools, other public buildings and in private properties. They are used in the performance of classical music, sacred music, secular music, and popular music. In the early 20th century, pipe organs were installed in theaters to accompany the screening of films during the silent movie era; in municipal auditoria, where orchestral transcriptions were popular; and in the homes of the wealthy.^[13] The beginning of the 21st century has seen a resurgence in installations in concert halls. The organ boasts a substantial repertoire, which spans over 500 years.^[14]

The organ is one of the oldest instruments still used in European classical music that has commonly been credited as having derived from Greece. Its earliest predecessors were built in ancient Greece in the 3rd century BC. The word organ is derived from the Ancient Greek ὄργανον (órganon),^[17] a generic term for an instrument or a tool,^[18] via the Latin organum, an instrument similar to a portative organ used in ancient Roman circus games.

The Greek engineer Ctesibius of Alexandria is credited with inventing the organ in the 3rd century BC. He devised an instrument called the hydraulis, which delivered a wind supply maintained through water pressure to a set of pipes.^[19] The hydraulis was played in the arenas of the Roman Empire. The pumps and water regulators of the hydraulis were replaced by an inflated leather bag in the 2nd century AD,^[19] and true bellows began to appear in the Eastern Roman Empire in the 6th or 7th century AD.^[5] Some 400 pieces of a hydraulis from the year 228 AD have been revealed during the 1931 archaeological excavations in the former Roman town Aquincum, province of Pannonia (present day Budapest), which was used as a music instrument by the Aquincum fire dormitory; a modern replica produces an enjoyable sound.

The 9th century Persian geographer Ibn Khurradadhbih (d. 911), in his lexicographical discussion of instruments, cited the urghun (organ) as one of the typical instruments of the Eastern Roman (Byzantine) Empire.^[20] It was often used in the Hippodrome in the imperial capital of Constantinople. A Syrian visitor describes a pipe organ powered by two servants pumping "bellows like a blacksmith's" as being played while guests ate at the emperor's Christmas dinner in Constantinople in 911.^[6] The first Western European pipe organ with "great leaden pipes" was sent from Constantinople to the West by the Byzantine emperor Constantine V as a gift to Pepin the Short King of the Franks in 757. Pepin's son Charlemagne requested a similar organ for his chapel in Aachen in 812, beginning its establishment in Western European church music.^[21]

From 800 to the 1400s, the use and construction of organs developed in significant ways, from the invention of the portative and positive organs to the installation of larger organs in major churches such as the cathedrals of Winchester^[23] and Notre Dame of Paris.^[24] In this period, organs began to be used in secular and religious settings. The introduction of organ into religious settings is ambiguous, most likely because the original position of the Church was that instrumental music was not to be allowed.^[23] However, by the twelfth century there is evidence for permanently installed organs existing in religious settings such as the Abbey of Fécamp and other locations throughout Europe.^[23]

Several innovations occurred to organs in the Middle Ages, such as the creation of the portative and the positive organ. The portative organs were small and created for secular use and made of light weight delicate materials that would have been easy for one individual to transport and play on their own.^[25] The portative organ was a "flue-piped keyboard instrument, played with one hand while the other operated the bellows."^[26] Its portability made the portative useful for the accompaniment of both sacred and secular music in a variety of settings. The positive organ was larger than the portative organ but was still small enough to be portable and used in a variety of settings like the portative organ. Towards the middle of the 13th century, the portatives represented in the miniatures of illuminated manuscripts appear to have real keyboards with balanced keys, as in the Cantigas de Santa Maria.^[27]

It's difficult to directly determine when larger organs began to be installed in Europe; however one of the first eyewitness accounts of organs is from Wulfstan of Winchester. This detailed account gives us an idea of what organs were like prior to the thirteenth century, when there are more records of large organs being placed in churches as well as their uses.^[28] In his account, he describes the sound of the organ: "among them bells outstanding in tone and size, and an organ [sounding] through bronze pipes prepared according to the musical proportions."^[28] This is one of the earliest accounts of organs in Europe and also indicates that the organ was large and more permanent than other evidence would suggest.^[29]

The first organ documented to have been permanently installed was one installed in 1361 in Halberstadt, Germany,^[30] The first documented permanent organ installation likely prompted Guillaume de Machaut to describe the organ as "the king of instruments", a characterization still frequently applied.^[31] The Halberstadt organ was the first instrument to use a chromatic key layout across its three manuals and pedalboard, although the keys were wider than on modern instruments.^[32] It had twenty bellows operated by ten men, and the wind pressure was so high that the player had to use the full strength of his arm to hold down a key.^[30]

Records of other organs permanently installed and used in worship services in the late thirteenth and fourteenth centuries are found in large cathedrals such as Notre Dame, where in the 1300s you can find documents of organists being hired to work for the church as well as records documenting the installation of larger and permanent organs.^[24] The earliest record is a payment from 1332 from the clergy of Notre Dame to an organist to perform on the feasts St. Louis and St. Michael.^[24] The Notre Dame School also shows how organs could have been used within the increased use of polyphony, which would have allowed for the use of more instrumental voices within the music.^[33] This shows that by this point in time organs were being fully used within church services and not just in secular settings. There is proof that organs existed earlier in the medieval period, based on the surviving keyboards and casings of some organs, however no pipes from organs survive from this period.^[34] Until the mid-15th century, organs had no stop controls. Each manual controlled ranks at many pitches, known as the "Blockwerk."^[35] Around 1450, controls were designed that allowed the ranks of the Blockwerk to be played individually. These devices were the forerunners of modern stop actions.^[36] The higher-pitched ranks of the Blockwerk remained grouped together under a single stop control; these stops developed into mixtures.^[37]