The relay is the generic term given to the electrical means by which the console is connected to the pipework and percussions. Theatre organs, usually being constructed on the extension principle, use the relays to cross-connect the pipework to the manuals and pedals without any backfeeding.
For example, if the 8′ Flute and 4′ Flute are drawn on the Accompaniment, and the 8′ Flute and 8′ String are drawn on the Great, then it is important that when notes are played on the Accompaniment, the 8′ String does not sound as well – as would happen if the manuals were connected directly to the windchests.
For more details on this, go to the page describing the extension and unit principles.
Theatre organ relays fall into two main categories; the traditional relays (electro-mechanical or electro-pneumatic) and today’s equivalents which utilise solid-state electronics to achieve the same result with more flexibility, fewer components and less wiring. The relative merits (and ethics) of replacing the old with the new continue to be debated.