tet·ra·chord / ˈtetrəˌkôrd/ • n. Mus. a scale of four notes, the interval between the first and last being a perfect fourth. ∎ hist. a musical instrument with four strings.

tetrachord (Gr. ‘four string’). Succession of four notes contained within compass of a perfect fourth. In Ancient Gr. mus. a tetrachord consisted of 4 notes descending through a perfect fourth in the order tone-tone-semitone (A–G–F–E) and joined together to form a series of eight-note modes. The modern diatonic scale is divisible into two tetrachords (C–D–E–F, and G–A–B–C).