Ed Hattrup asked how to determine the location of "nodal holes" to
make the pipes sound the twelfth (third harmonic), the octave and
other pitches.

An open pipe overblown to the octave has the hole drilled at about 43%
of the length of the pipe, measured from the languid to the top of the
tuner in a slot, or to the top of the pipe if there is a scroll over
the top.

Double overblown pipes are quite rare.  I don't know of any organ pipes
that are made to play the same pipe at two different overblown pitches,
if you were thinking of that.  The manner of blowing would also have to
be adjusted as it is on a mouth blown flute or recorder.

Overblown pipes are considered the most challenging to voice well.
It can be done either for the tonal effect of the different harmonic
structure of the note, or to drive a pipe to a very high output.
If all of the parameters -- wind pressure, toe hole, flue, and cut-up
-- are extremely precise, the desired over-blowing can be achieved
without nodal holes.

John M. Nolte - Nolte Organ Building & Supply, Inc.
Milwaukee, Wisconsin