a "coupler" allows the stops of one division to be played from the keyboard of another division. for example, a coupler labelled "swell to great" allows the stops drawn in the swell division to be played on the great manual. this coupler is a unison coupler, because it causes the pipes of the swell division to sound at the same pitch as the keys played on the great manual. coupling allows stops from different divisions to be combined to create various tonal effects. it also allows every stop of the organ to be played simultaneously from one manual.