四度音階,四弦琴
Tetrachord \Tet"ra*chord\, n. [L. tetrachordon, Gr. ?, from ? four-stringed; te`tra- (see {Tetra-}) + ? a chord: cf. F. t['e]trachorde.] (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.