Tetrachord
Pronunciation : Tet"ra*chord
Part of Speech : n.
Etymology : [L. tetrachordon, Gr. Tetra-) + t?trachorde.] (Anc. Mus.)
Definition : Defn: 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.
Source : Webster's Unabridged Dictionary, 1913