Octave-Sounding Pitches
Unison- or octave-sounding stops sound either at the normal pitch of the key being played (unison-sounding), or at some octave above or below the key (octave-sounding). All pitch levels described thus far--32', 16', 8', 4', 2', and 1' stops--belong in this category. Certain octave-sounding stops are also sometimes called foundations.
In contrast to unison or octave-sounding pitches, which are designated by whole numbers (32', 16, 8, etc.), mutations are stops that include fractions in the pitch designation (such as 2 2/3' or 1 3/5'). These sound at some pitch other than the unison or octave (most commonly at the fifth or third) of the key being played.
