However, the stops on the Great will still sound if keys on the Great manual are played. Some organs also have a Unison coupler which disconnects the manual from playing stops on its corresponding division. In the Great to Pedal example, if a Unison coupler was turned on, the Pedal would still play stops from the Great division but the Great manual would not be able to play the stops in the Great division. When the Unison is used with other couplers, it can maximize the different combinations of stops available. The choice of stops and couplers which the organist uses in a piece of music is the registration.
Some composers specify the stops to be used.
If the organ does not have the same exact stop names that the composer wants, the organist substitutes a similar stop. Open Diapason, an 8' Montre can be substituted. The Montre does have a different tonal quality than the Open Diapason but they are close enough so that the musical effect is still close to what the composer wanted. However, substitution does not always work well. Some organ works use so many stops that they can only be played on large organs.