Mutation couplers work in an identical way to inter-manual couplers or intra-manual couplers, except that instead of playing the same note elsewhere (on a different manual or an octave higher or lower on the same manual), they play a different interval
For example, a Solo to Great fifth (or ‘quint’) coupler will play a note on the Solo manual a fifth interval above the note played on the Great manual: hence with this coupler drawn, if middle C is played on the Great manual, G above middle C plays on the Solo, if A is played on the Great, then E plays on the Solo, and so on.
The best known example are found on the Wurlitzer organ of the Tower Ballroom, Blackpool. These were fitted to the organ during the 1950’s as a means of providing more incisive solo registrations and, it is said, were inspired by a visit Reginald Dixon made to the Moller organ then in the BBC studio at Hoxton, which was also equipped with mutation couplers.
The distinctive sound of these couplers on the Tower organ is due to a mix of mutation and unison inter-manual couplers, coupling the Great to the Solo at various pitches as follows:
Middle C played on the Solo plays:
Great to Solo
Middle C on the Great
Great to Solo 4′
C above Middle C on the Great
Great to Solo sub-quint
G below Middle C on the Great
Great to Solo sub-tierce
E below Middle C on the Great
Such couplers have to be used with care and discretion, most usually in single notes.