Tetrachord n. (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.
... compared to a builder engaged in some great design, who could only dig with his hands because he was unprovided with common tools; or to some poet or musician, like Tynnichus (Ion), obliged to accommodate his lyric raptures to the limits of the tetrachord or of ... — Timaeus • Plato