Tetrachord

Tet″ra‐chord (?), n. [L. tetrachordon, Gr. �, from � four-stringed; τέτρα- (see Tetra-) + � a chord: cf. F. tétrachorde.] (Anc. Mus.) A scale series of four sounds, of which the extremes, or first and last, constituted a fourth. These extremes were immutable; the two middle sounds were changeable.