sometimes, a single rank of pipes may be able to be controlled by several stops, allowing the rank to be played at multiple pitches or on multiple manuals. such a rank is said to be "unified" or "borrowed". for example, an 8′ diapason rank may also be made available as a 4′ octave. when both of these stops are selected and a key (for example, c′) is pressed, two pipes of the same rank will sound: the pipe normally corresponding to the key played (c′), and the pipe one octave above that (c′′). because the 8′ rank does not have enough pipes to sound the top octave of the keyboard at 4′ pitch, it is common for an extra octave of pipes used only for the borrowed 4′ stop to be added. in this case, the full rank of pipes (now an "extended rank") is one octave longer than the keyboard.