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 ground-tone 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,...