A Dictionary of Music and Musicians/Sesquialtera

SESQUIALTERA. A compound organ stop consisting of several ranks of pipes, sometimes as many as five. Various combinations of intervals are used, but they only represent different positions of the 3rd, 5th, and 8th of the groundtone in the third or fourth octave above. The Sesquialtera thus gives brilliancy to the tone by reinforcing these upper partials.

The origin of the term Sesquialtera, as applied to an organ stop, is rather obscure. In the list of ratios given by Boethius, at the close of the 5th or beginning of the 6th century, which were exactly reproduced by almost every writer on music up to the 16th century, the term proportio sesquialtera signifies numbers having the ratio 2:3; the term therefore is really applicable to all stops having pipes at an interval of a fifth from the groundtone, such as the Quint, Twelfth, Larigot (nineteenth), etc. As stated above, the Sesquialtera organ stop does actually contain pipes having this relation, only, (it also contains pipes having the ratio 5:4 the tierce), which Boethius called a proportio sesquiquarta. On the whole it may be safely said that the word Sesquialtera was originally used for the purpose of showing that the stop contained pipes having ratios other than 2:1, or other than an octave-series.

[ J. S. ]