In musical theory
, a tetrachord
is a series of four diatonic
tones encompassing the interval
of a perfect fourth
Two similar tetrachords, a tone apart, form the diatonic major scale.
In musical set theory, a tetrachord is a collection of four pitch classes, often one of the three ordered tetrachords in a tone row or set form. Tetrachords may be used to create derived rows and invariance.