Tetrachord
Tetrachord , noun
[Latin tetrachordon, Greek {not transcribed}, from {not transcribed} four-stringed; te`tra- (see Tetra-) + {not transcribed} a chord: compare French tétrachorde.]
(Anc. Music) 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.