Unison Pitch (Aequal)
Term for organ stops sounding at normal, eight-foot pitch, as opposed to stops sounding higher or lower.
What it means
Aequal, from the Latin aequalis meaning "equal," is organ terminology for stops that sound at normal, that is eight-foot, pitch. A stop at unison pitch reproduces exactly the note written for the key pressed, without shifting it an octave up or down.
Unison pitch thus contrasts with higher- or lower-sounding stops, such as four-foot stops sounding an octave higher, or sixteen-foot stops sounding an octave lower. The foot number traditionally denotes the length of a stop's longest pipe and, with it, indirectly its pitch register.
In the notation
Unison pitch is not a notation symbol but a registration term. Organ stop labels usually show the number 8' to mark unison pitch, as opposed to 4', 2', or 16' for higher- or lower-sounding stops.
Musical use
The distinction between unison and octave stops is fundamental to organ registration practice. Unison stops usually form the tonal foundation of a registration, on top of which higher and lower stops are added to give the sound more fullness, brilliance, or weight.
Other keyboard instruments, such as the harpsichord or harmonium, occasionally distinguish unison from octave stops as well, following the same principle: a stop either sounds at written pitch or shifted by one or more octaves.
Performance practice
When registering an organ, it usually helps to start with a unison stop as the foundation and then add octave or sub-octave stops depending on the desired richness of sound. This keeps track of exactly which pitch register the resulting sound actually occupies.