The pipe organ is a musical instrument that prouces sound when pressurized air (referred to as wind) is driven through a series of pipes. The admission of wind into the pipes is controlled by a keyboard. A pipe sounds when a key is depressed on the keyboard, allowing wind to pass through the pipe. Modern organs usually include one or more keyboards playable by the hands and one keyboard playable by the feet. Each keyboard controls a certain number of pipes. The smallest portable organs may have only a few dozen pipes and only one keyboard, while the largest organs may feature over 20,000 pipes and seven keyboards. The organ's continuous supply of wind allows it to sustain sound for as long as a key is depressed. This is unlike other keyboard instruments such as the piano and harpsichord, whose sound begins to decay immediately after the key is struck.