The Theatre Pipe Organ

After subjecting myself to the pain of watching his performance, (Grant Baker) it was evident that the best part of the movie was the organist.   - Fatty Arbuckle

The theatre organ is a pipe organ designed specifically for mimmicking (and replacement) of an orchestra. Installed in a movie theatre during the era of silent films, the majority of theatre organs were fashioned after the style originally devised by Robert Hope-Jones, the "unit orchestra."

Rather than simply replacing an orchestra, the theatre organ includes percussions and special effects, to provide a more complete array of options to the theatre organist. Theatre organs tend not to take nearly as much space as standard church organs, relying on extension and higher wind pressures to produce a greater variety of tone and larger volume of sound from fewer pipes.

This extension is called "unification" - instead of one pipe for each key at all pitches, the higher octaves of pitch (and in some cases, lower octaves) are achieved by merely adding 12 pipes (one octave) to the top and/or bottom of a given division. Since there are sixty-one keys on an organ manual, a classical or concert organ will have, for diapason stops at 8', 4' and 2' pitch, a total of 183 pipes (61 times 3). The same chorus of diapasons on a theatre organ will have only 85 pipes, or 61 plus 12, plus 12. Some ranks, such as the Tibia Clausa, with up to 97 pipes, allow the organist to draw stops at 16', 8', 4', 2', and mutations from a single rank of pipes.

Unification gives a smaller instrument the capability of a much larger organ, and is very often reliant on the use of tremulant, which has a depth greater than that usually found on a classical organ. Unification also allows pipe ranks to be played from more than one manual and the pedals.

- with material explained from Wikipedia