Relief Editor wrote:
    My editing may have caused some confusion here.  I believe John is
    referring to a situation where wind is being supplied to all the
    pipes in question, not just to one pipe.  John?

Perhaps I could have explained it better.  Take two pipes standing in
close proximity to each other, and pitched at the same note.  Pipe B is
slightly out-of-tune with pipe A.  Pipe A is playing; pipe B may be
playing or not.  The resonant frequency of the body of pipe B may
influence the playing frequency of pipe A, and so pulls its pitch
towards that of pipe B.

In practice, in a church organ, this is the reason the 4' principal
rank is never placed adjacent to a 4' harmonic flute.  When tuning is
in progress the principal would be influenced by the flute pipes
alongside, rather like magnets.  There is always an 8' rank placed
between them.

Take the case of a Mortier dance organ, a situation where two similar
ranks are playing.  The two ranks of violins playing together are
actually tuned by pulling each other "out" very slightly, but not so
much that they start oscillating like a celeste.  This has the effect
of producing more of an edge to their tone, which is a distinctive
feature of these Belgian organs.

Another example: two ranks of violins, one being tuned celeste with the
other, may be placed just too close for comfort (a design oversight?).
Detuning the celeste pipes _should_ produce the familiar undulating
sound, which gradually increases in frequency the more the celeste pipe
is detuned (usually sharpened).  I have found in some cases that this
does not happen.  Instead the pitch of the original "straight-tuned"
pipe is pulled out with the celeste pipe until it cannot hold on any
longer.  It then jumps to the normal celeste sound, but far too fast
for good tuning.  When this happens -- and it is quite rare -- a solu-
tion is to turn one of the pipes around 180 degrees, so the mouths are
pointing away from each other.  This reduces the "pull" to a negligible
effect.

I'm sorry for the rather lengthy explanation, but this phenomenon can
be demonstrated easily by using an electronic tuner, and watching the
variations as the pipes are tuned.  Try changing the tuning of a nearby
pipe that is not playing, while watching the tuner's response to the
pipe that is playing.

Regards,
John Page,
UK