The name Sesquialtera and its variants most commonly and properly denote a compound flue stop of two unbroken diapason ranks, speaking the 12th and 17th of whatever harmonic series the stop is intended to support. The most common examples support the 8' harmonic series, and thus have ranks at 2-2/3' and 1-3/5' pitch. (See Twelfth and Seventeenth.) Historically, the composition of the Sesquialtera has not been so cut and dried. Williams dates the earliest German examples from the 16th century; these consisted of the two canonical ranks, and were used for both solos and choruses. Starting in the late 17th century, an additional rank or two was sometimes added, usually the 15th. Adlung, writing in the early 18th century, describes it as "everywhere and always". By the late 19th century, however, the original two-rank Sesquialtera had become rare, according to Hopkins & Rimbault. As a single stop, the Sesquialtera was very rare in early French instruments, according to Douglass. Single-rank mutations at 2-2/3' and 1-3/5' were common enough, but were always of wider scale than the German stops. Grove dates the English Sesquialtera from the late 17th century, where it started as a bass for the Cornet, and as a chorus mixture. The most common composition was 17-19-22. By the 19th century it had become the only mixture in the entire organ, but often lacked a third-sounding rank, especially when such a rank was available as a separate mutation. Wedgwood, writing at the turn of the 20th century, states that the name was then falling into disuse, and in 1923 Bonavia-Hunt called it "practically obsolete".
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