A Dictionary of Music and Musicians/Mutation stops
MUTATION STOPS, in an organ, are those registers which do not produce a sound agreeing with the name of the key pressed down, but either the perfect fifth or the major third to it, as G or E on the C key. The former are called fifth-sounding, or Quint stops; the latter third-sounding, or Tierce stops. The proper relative size of the largest fifth-sounding stop is one-third that of the Foundation stop from which it is deduced; as 10⅔, 5⅓, or 2⅔, from the 32, 16, or 8 feet stops respectively. The largest Tierce-sounding stops are one-fifth the size of the Foundation stops from which they are deduced; as 62⁄5, 31⁄5, and 13⁄5 feet respectively. The third-sounding rank on the manual has been much more sparingly used since the introduction of Equal Temperament, as it does not sound agreeably with that system of tuning; and an additional rank of pipes consequently becomes available for some other purpose.The only Mutation stop in use in England previously to the arrival of Smith and Harris (1660) was the twelfth (2⅔ feet). After that date the Tierce (13⁄5 foot), Larigot (1⅓ foot), and their octaves (among the small Mixture ranks) became not uncommon.
[ E. J. H. ]