A variety of keyboard instruments.

The first organ was probably a Ancient Greek water organ, a type of pipe organ (which use wind moving through pipes to produce sounds) blown by air, where the power source pushing the air is derived from pumped water.

Later pipe organs used bellows to produce wind and by the 12th century were beginning to evolve into complex and often quite large instruments capable of producing different timbres and significant volume and consequently ideal for accompanying choral music, especially in a church.

Non-piped organs such as the reed organ (including the harmonium) use air to excite free reeds, like the accordion and harmonica ('mouth organ') .

Modern electronic or digital organs generate electronically produced sounds which can be made to resemble those produced by more traditional pipe or reed organs. They are a low-cost substitute for pipe organs and in popular music (rock, blues and jazz) are an alternate keyboard sound to the piano.