Introductory text to the very interesting “Stories of London” website which has a wealth of material on theatre and pipe organs.
A Theatre or Cinema Organ is a distinct type of Pipe Organ developed for a specific purpose, which was to entertain an audience and grew to be a unique instrument.
Unlike the Pipe Organ where the pipes were built into a decorative casing and remained in full view of the audience, the pipes of the Theatre Organ were placed in chambers and hidden by ornate plasterwork grilles, but allowed the sound to pass into the auditorium.
Only the console was visible to the audience and was either mounted on a side wall or else on a platform that rose from the orchestra pit to a position of prominence on the stage where it was played. The organ was a Unit Orchestra in that the various sets of pipes (Ranks) imitated the various tone families of a full orchestra (strings, woodwinds, brass etc).