Pipe organs are complex musical instruments, with hundreds or thousands of pipes of varying size and pitch. Controls on the console of the organ called stops select which pipes are used; different combinations of stops can change the timbre of the instrument considerably. Many pipe organs are mammoth instruments that are part of the church building itself.
Pipes may be classified in a number of different ways: by the material they are made of (wood or metal), by the mechanism of sound production (flue pipes vs. reed pipes), and by the construction of the ends (open or closed). Each variation results in a different timbre.
An organ pipe produces only one pitch, so there must be at least one pipe for each controlling key or pedal. Thus, a keyboard with 61 notes would require 61 pipes. A complete set of pipes producing different pitches of one timbre is called a rank. The pitch produced is a function of the length of the pipe, and many timbres are associated with ranks pitched some multiple of octaves apart: thus an organ stop may have similar names with the addition of a length in feet indicating the pitch: a 16' stop produces pitches an octave below that of an 8' stop, an 8' stop produces pitches an octave below that of an 4' stop, and a 4' stop produces pitches an octave below that of an 2' stop.
Some timbres require more than one pipe per key. This is often reflected in the name given to the stop as a Roman numeral: thus a stop called "Cornet V" on a 61 note manual (this is the usual number on U.S. organs) would have 5 x 61 = 305 pipes.
The pipe organ has at least one keyboard, with 2-5 keyboards being the most common configuration. Each keyboard is called a manual, so that an organ with four keyboards is said to have four manuals. Most pipe organs also have a set of keys played by the feet called "pedals". The manuals, pedals and stop controls are gathered together in a functional piece of furniture called a "console".
From the time of the organ's invention by the ancient Greeks until the 19th century, pipes were originally located within a cabinet or "case", with the console and related mechanism built in. The valves under the pipes were connected by mechanical linkages to the keys, so that the organist's fingers actually provided the energy to open the valves. This system is known as "mechanical (or "tracker") key action",
With the invention of electrical and pneumatic control systems in the late 19th century, organ pipes were often located remotely from the console in special rooms called chambers. In the 1920s and '30s, there was a revival of interest in historic organs, and organ builders once again began building organs with mechanical action. Today, both electric action and mechanical action pipe organs are built.
The largest pipe organ ever built is the Main Auditorium Organ in Atlantic City Convention Hall.
Electronic organs such as the Hammond Organ were originally developed as imitations of pipe organs.
Notable organ builders: