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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. Originally tetrachord referred to the four strings (tetra - chord) of Greek lyres of which two, the outside or lowest and highest, were fixed (in a just, 3:4, perfect fourth) and two, the inside or middle strings, were movable.
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.
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