Mutation stop [overtone stops]

(Ger. Aliquotstimmen).

In modern organ usage, mutations are understood to mean stops of one rank of flue pipes whose sounding pitch is not the same note as the key which plays them, whether they are made to a scale similar to the Open Diapason of Principal (e.g. Twelfth, Seventeenth, Nineteenth) or wider (e.g. Grosse Quinte, Grosse Tierce, Quinte, Tierce, Larigot), and whether stopped, chimneyed, open or conical in construction. Romantic organs occasionally include mutation stops derived from contemporary studies of harmonics, e.g. the Septième (11/7'; where B sounds from a C key), and some post-Orgelwebegung organs have mutations unknown to classical organs, such as the None (8/9'; sounding D from a C key), sometimes combined in ‘aliquot’ or ‘colour’ Mixtures. From the Renaissance onwards, organ makers have also exploited the fact that a stopped pipe can be made to sound its first harmonic (a 12th) at about the same strength as its fundamental tone (Quintade, Quintadena, Quintatön), but these stops have been employed for their characteristic sound, and not (usually) as part of a series of mutation stops related to the harmonic series. True mutation stops are never used without a suitable foundation, and the organist can combine them in various ways to imitate the effect of some wide-scaled Mixtures such as Cornet, Sesquialtera and Tertian.

Historically, ‘mutation’ (Ger. Mutationen; Sp. mutaciones) meant much the same as ‘mixture’ did in 18th-century England: a combination of stops (registration) or the stops themselves (Franciscan church, Barcelona, 1480; J.B. Samber: Continuatio ad manuductionem organicam, 1707: Mutationen was a synonym for Stimmen). In the early French classical organ, stops outside the plein jeu were so called, perhaps because they could be combined freely.

For the use of the term on pianos, see Pedalling, §1; see also Organ stop and Mixture stop.

MARTIN RENSHAW