The early organ-builders to obtain variety of tone divided the pipes into groups placed in various positions, each playable from a separate keyboard, and this practice prevails to this day. An average church organ will contain three or four wind-chests, each with its quota of pipes and designated as follows:
1. The Great organ, consisting of the front pipes and other loud-speaking stops. Back of this and usually elevated above the level of the Great organ pipes is
2. The Swell organ, all the pipes of which are contained in a wooden box with Venetian shutters in front, the opening or closing of which modifies the tone; below the Swell box is placed
3. The Choir organ, containing soft speaking pipes suitable for accompanying the human voice; and back of all or on the sides is
4. The Pedal organ, containing the large pipes played by the pedals.
Larger instruments have still another wind-chest called the Solo organ, the pipes of which are very loud and are usually placed high above the Great organ.
In some large English organs, notably that in the Town Hall of Leeds, a further division was effected, the pipes of the Great organ being placed on two wind-chests, one behind the other. They were known as Front Great and Back Great.
The original reason for dividing a church organ in this manner seems to have been the impossibility of supplying a large number of stops with wind from a single wind-chest.
It will thus be seen that our average church organ is really made up of three or four smaller organs combined.
The wind-chest is an oblong box supplied with air under pressure from the bellows and containing the valves (called pallets) controlling the access of the wind to the pipes. Between the pallet and the foot of the pipe comes another valve called the slider, which controls the access of the wind to the whole row of pipes or stop. The pallet is operated from the keyboard by the key action. Every key on the keyboard has a corresponding pallet in the wind-chest, and every stop-knob operates a slider under the pipes, so that both a slider must be drawn and a pallet depressed before any sound can be got from the pipes. The drawings will make this plain.