Mutations

MUTATIONS are stops that sound at some pitch other than the unison or octave above or below the key being played. The most common mutations sound a perfect fifth or a major third (plus one or two octaves) above the key being played. Mutations are easily identified by the fraction in the pitch designation, the most common being 2 2/3', 1 3/5', and 1 1/3'.

The opposite of mutations are unison or octave-sounding stops (also called foundations): 32', 16', 8', 4', 2', and 1' stops. They sound either at the same (“normal” or “concert”) pitch of the key being played (unison-sounding), or at some octave above or below the key (octave-sounding).