The organ is a complex wind instrument that employs one or more keyboards to operate valves that admit air into a series of individual pipes, which make the sound. Organs are related to the syrinx, or panpipes, with its row of individual pipes that are blown directly by the musician, and the bagpipe, with its bag storing and providing air to the pipes.
The ancient Greek engineer and theoretician Ctesibius (second-century B.C.) is credited with the invention of the organ, or hydraulis. The hydraulis was so called because the air pressure was controlled by a reservoir of water, and the player would use fingers or fists to depress levers or sliders to uncover the base of the pipe to admit the air. These organs could play long, sustained notes to accompany singers, but as organ music became more complex and polyphonic, keyboards were substituted for the sliders to allow the player greater facility.
Portative organs were small and could be played on the musician’s lap or set on a table. They were popular from about 1100 to 1650, and can be seen in works of art (Gubbio Studiolo, 39.153). They could be played by one person operating a bellows with the left hand and the keyboard with the right, or by two people, one pumping the bellows and the other operating the keyboard (Coronation of the Virgin, 1975.1.38).
Arkenberg, Rebecca. “Renaissance Organs.” In Heilbrunn Timeline of Art History. New York: The Metropolitan Museum of Art, 2000–. http://www.metmuseum.org/toah/hd/reno/hd_reno.htm (October 2002)