To answer Ray Finch's question, 'octave coupling' is simply "getting
two for the price of one".  In other words, one note is played, but two
notes sound.  Actually, there are two basic ways to accomplish octave
coupling.  One is mechanical and the other is pneumatic.

Mechanical coupling always involves some sort of a device that (when
activated) senses that a note is being played and then automatically
plays a note that is an octave higher or lower than that note --
usually a lower note in the bass register, and higher in the treble
register.  Such devices are very common in reed organs and they can
be turned on or off by pulling out or pushing in a stop.  These are
typically labeled Bass Coupler or Treble Coupler.

In roll-operated instruments, pneumatic coupling is accomplished most
easily by placing a "T" connector in line with any given note, such
that activating that note will also activate another note that is one
octave away from the primary note.  In most cases, pneumatic octave
coupling is "hard wired" into the system.  Octave coupling in pneumatic
instruments is normally done to extend the range of a short keyboard
(64 or 65 note unit) so that all 88 notes on the music roll will play
at least one note on the instrument.

There is one another type of octave coupling, but it is associated
with music roll manufacturing.  On the machine that is used to arrange
the music roll, a device can be activated that allows the arranger to
add in notes that are either an octave above or below any given note.
I've never personally seen how this is accomplished, but I would
imagine that it is very similar to the mechanical device used in
a reed organ.

In all cases, the basic purpose of octave coupling is to make the
music sound "fuller" than it was in its original format.

Musically,

John A. Tuttle