The organ, in orchestral terms, is usually accepted as the Pipe Organ. It consists of between 1 and several hundred pipes, each one as a single pitch, which is controlled from a keyboard, or multiple keyboards, called manuals. Each rank of pipes has a unique tone quality, as as pipes are single pitch, each rank has a pipe for each pitch. Each manual control several ranks of, more or less, 61 pipes. Usual a typical organ as an example, there are generally two manuals, with the upper called the Swell Organ, and the lower called the Great Organ. More or less manuals can be found on larger or smaller instruments, so our description is general, and Organ music is usually written over three staves. There is another keyboard played with the feet called the Pedal Clavier, and this particular manual is indicated by the bottom, or 3rd stave in notation. A Pedal Clavier rank usually consists of 32 pipes.
It would be difficult for us to list all the variations of Organ construction, as many Organs are custom built, not only for musical reasons, but location as well, dependent on the size of the venue, and the dimensions within which a Organ can be built.
Ranks of pipes, as written above have different tone qualities. Some ranks will be loud and assertive, while others while be soft, and possible ethereal in tone. The Organ manuals are constructed in such a way that these ranks are accessible by means of a series of stops, or levers. With these devices, the performer can mix ranks to produce a wide array of tonal variations, played simultaneously from the same manual. A stop is usally associated with a single rank, and when the stop is pulled out, or active, it is considered "drawn".
In earlier classical music, many works were written for Organs in churches and cathedrals, and there is a general perception that the Pipe Organ is somehow assumed to be...religious in nature. This is far less the case in 19th and 20th Century orchestral music, and Pipe Organs began to find their way into concert venues and grand halls, and this inovation led to many composers of the time using the Pipe Organ in an entirely different context. As a full concert instrument, the Organ provides many tone colours, and combinations, to enhance the composer's toolbox, and with the continued contruction of Pipe Organs in modern venues, the modern composer should be confident in writing work for this instrument, if live performance is intended.
We will add more here in the future, but due to the complexity of the instrument, users are encouraged to explore external resources available to them now, and gain a much greater understanding of writing for the Organ.