Tetrachord

n. [ L. tetrachordon, Gr. &unr_;, from &unr_; four-stringed; te`tra- (see Tetra-) + &unr_; 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. [ 1913 Webster ]