A Dictionary of Music and Musicians/Coupler
COUPLER. All modern organs are provided with mechanical appliances called 'couplers.' These useful adjuncts are of two general kinds 'manual couplers' and 'pedal couplers.' (1) The former operate in one of three ways: either by taking down on one manual the key corresponding to that played on another, in which case it is a ‘unison coupler’; or by taking down the octave above the note pressed down, when it forms an ‘octave coupler,’ sometimes incorrectly called a 'super-octave coupler'; or by operating on the octave below, forming a 'sub-octave coupler.' The octave and sub-octave couplers sometimes act on the manual on which the note is struck. The couplers are put in action by draw-stops inscribed according to circumstances—as 'Swell to Great,' 'Great to itself,'—or by pedals. Manual couplers date back at least as far as 1651, when Geissler's organ at Lucerne was completed; which, according to the account formerly existing over the keys, contained 'several registers, whereby one may make use of the three manuals together, or of one or two of them separately.'
(2) A pedal coupler attaches a particular manual to the pedal-clavier; and by bringing the lower 2½ octaves of the compass of the manual under the control of the feet, produces the effect of a third hand on any manual required.