Stops

The Pipe Organ has stops which normally control a single rank of pipes - and you find different types, diapasons (which are the basic sound of the organ), flutes, or reeds.

The little number that appears describes the pitch. If you play middle C on an 8 foot stop you get middle C. 4 foot stops would give you an octave above, 2 foot two octaves above -16 feet an octave below, etc.

Bringing all those combinations together, and more produces the "registration" for a particular piece which gives it the special sound.