Sesquialtera
An organ stop combining a fifth and a third.
What it means
The sesquialtera is a compound organ stop usually consisting of two ranks of pipes: a fifth (typically 2⅔ feet) and a third (typically 1⅗ feet), which sound together whenever a note is played.
The name comes from the Latin term for the ratio 3:2, which originally described the fifth in this stop combination.
Musical use
The simultaneous sounding of a fifth and a third above a fundamental creates the impression of a complete major triad, giving the sesquialtera a characteristically bright, overtone-rich, slightly reedy sound quality.
In the North and Central German organ-building tradition of the seventeenth and eighteenth centuries, the sesquialtera was a frequently used solo and reed-supplementing stop, for instance for chorale preludes where a melodic voice needs to stand out.
The sesquialtera should be distinguished from the mixture, which consists of considerably more ranks with a changing composition across the range, and from simple mutation stops such as a plain fifth, which supply only a single overtone rank without the characteristic addition of the third.
Performance practice
Use the sesquialtera deliberately where a melodic voice should stand out, for instance in a chorale prelude, rather than blending it into the full organ. Combine it sparingly with other stops, since its striking sound can quickly become overpowering.