The organ has often been called “the king of instruments.” Owing to its great age (more than 2000 years), size, and mechanical complexity, this nickname is well deserved. It is also the instrument most affected by the invention of electricity and other modern technologies. Today there are church organs, theater organs, home organs, pipe organs, electronic organs, and digital organs. The most prestigious organs have many rows or “ranks” of pipes. Other types produce their sound through electronic or digital circuitry. Many pipe organs have keyboards where wind assists the movement of the keys (so-called “pneumatic” action or “electro-pneumatic” action), though many fine church instruments–old and new–still use the mechanical, “tracker” action. But even the most well preserved historical organs today have their wind power supplied by electricity, where centuries ago the employment of people to pump the bellows would have been necessary.
The basic principal of the organ is that air, under pressure, is forced through a pipe. The wind supply, whether it is created by an electric blower or from pumped bellows, is stored in a wind chest or reservoir. As the player presses a key, the wind is released from the chest into the pipe, producing the sound. Traditional organ pipes are of two general categories, flue pipes and reed pipes. In flue pipes, the sound is produced from the vibration of a column of air. In reed pipes the sound is produced from the vibration of a brass tongue (or reed) mounted inside the pipe. Sub-categories exist of both types of pipes, and are differentiated according to the materials used in the construction of the pipes, their shapes, and their scale (the mathematical relationship between the thickness and length of the pipes). Types of flue pipes include principals, strings and flutes, and types of reed pipes include chorus reeds and solo reeds. Some of the pipes are made from metals including tin, lead, copper and zinc, and other pipes, especially some flue pipes, are made from wood. The shapes of the pipes can be cylindrical, square, tapered, open on top, or closed on top.

The subject of organ stops is very difficult to follow. The word stop is used for the levers or knobs or buttons on organs that engage or disengage the various ranks of pipes. Each organ has certain stops (or ranks of pipes) associated with each keyboard and the pedal board, and often couplers are available to give access on one keyboard to pipes associated with another. Since the 16th century, every organ has had an assortment of stops that varies from instrument to instrument. Many of the stop names were developed many centuries ago in different European countries. Therefore, the names of the stops on any given organ may be derived from different languages and often from archaic instruments that are no longer familiar member of the orchestra. So for example, the stop called Open Diapason on some organs and Principal on others, is the same stop. The name Diapason is descended from English organs and the name Principal is descended from German organs. Likewise, the stop called Stopped Diapason (designating a pipe with a closed top) on organs using English terminology is generally equivalent to the stop called Gedackt on instruments using German terminology. It is very common to find a co-mingling of languages referring to the stops on a single instrument, especially on very large and complex instruments.

Every time an organist plays the organ, he or she needs to select what combination of stops are appropriate for the music being played. This process is called registration, and given the individuality of each organ and its acoustical setting, a performer may chose to register different stop combinations when playing the same music on different organs.

Although ancient and medieval organs seem to have had entirely secular uses, from about 900 to 1200 the organ was exclusively connected in Europe to the Christian church. During the Baroque period, there were small portative organs that could be moved around and were used interchangeably with harpsichords in chamber music. It was only with the Protestant Reformation that the organ became an instrument to accompany congregational singing. After the anticlericalism that was part of the French Revolution, the organ in the 19th century was built with the goal of duplicating the symphony orchestra, an organ that is known as the Romantic organ in contrast to the Baroque organ of earlier times. Not only were stops intended to replecate the sound of string instruments introduced in greater number into the organ, but with the Industrial Revolution many technical modifications were made in the way the instrument was constructed. The most important French builder of the period was Cavaillé-Coll and his instrument at St. Sulpice in Paris inspired the great 19th century French organ composers, especially Charles-Marie Widor who composed huge work that he called Organ Symphonies.

The invention of the silent motion picture gave rise to the theater organ. An Englishman, Robert Hope-Jones, is credited as the inventor of the theater organ, and in the United States he collaborated with the Wurlitzer Company. Once “talkies” supplanted the need for theater organs to accompany movies, organs were used on the radio, which lead to the development of the fully electronic organ developed by Laurens Hammond. The name Hammond became associated with the highly portable electronic organ in the same way that Wurlitzer had been associated with the huge theater organs. The largest theater organ ever built (as confirmed by the Guinness Book of World Records) is the Atlantic City Convention Hall organ with 449 ranks of pipes, 336 stops and 33,112 individual pipes.