The Principal of the Matter

The foundation stops of the organ--the real "guts" of the organ sound, if you will (and many organists won't!)--are known variously as the "principles" or the "diapasons" or as "montre." This is because the organ speaks many languages.

The organ at our church is a German speaking instrument, so the stops there are known as principles. Their sound is somewhat "fluty," but they are much louder, and have a more aggressive tone than the flutes. This is because the width of the pipes (flue pipes, remember?) is much narrower than the pipes used for flute stops. Organ builders refer to this as the "scale" of the pipes. Generally a narrower scale results in a less rich sound. As for the volume? I think we owe some of that to Mr. Bernoulli and his principle but I haven't quite been able to figure it out given the technical nature of the Wikipedia articles! Besides, the organ builders got there several centuries ahead of either him or Newton.

In any case, foundation stops are necessary for accompanying the congregation during hymns, and since they aren't expected to be very quiet, they are always placed out front, where they are "on display" (which is what "Montre" means in French) and therefore can't be controlled dynamically by the shutters which separate the pipes out front from the ones in the pipe room behind (see my previous article if you want to crawl around in the pipe room).

not actual size

When you sit in a pew and stare up at the organ pipes, the ones you are looking at are the foundation pipes, or the principles. The rest of the organ pipes, some four-fifths, I would estimate, are the ones you can't see back in the pipe room.

I estimate our organ has some 1,000-1,200 pipes and this was recently affirmed by the head of our organ building firm in town. I don't have an exact number (shame on me) but you get the idea. I can't exactly take it home with me in the evenings to practice.