Couplers
Sub- and supercouplers have the effect of shifting the range of a stop one octave higher or lower. Using a subcoupler calls for twelve additional pipes for the bottom octave, and a supercoupler calls for twelve additional pipes for the top octave. However, an organ builder normally provides only the exact number of pipes required for a rank to function as intended, without the subcouplers or supercouplers (usually 61 or 58 pipes). Unless the extra pipes are provided, the effect of the subcoupler will not be heard when playing keys in the bottom octave, and that of the supercoupler will not be heard when playing in the top octave.
If an organ includes the twelve extra pipes in the extra bottom or top octave, this set of pipes is called an “extension.” Extensions are rather commonly provided in the lowest octave so that a rank may be used as a 16' Pedal stop. Extensions are sometimes provided in both the highest and lowest octaves, resulting in a rank of up to 85 pipes.
TRY IT: Pull Bourdon 8' on the Swell, and Swell to Swell 16'. Now play from middle C down to the lowest C. If one rank drops out in the bottom octave, there is no lower extension.
