hexachord

Noun

hex=a=chord

CIDE DICTIONARY

hexachord, n. [Hexa- + Gr. string, chord: cf. F. hexacorde.].

A series of six notes, with a semitone between the third and fourth, the other intervals being whole tones. [1913 Webster]

hexachord, n. a diatonic series of six notes with a semitone between the third and fourth, used at three different pitches in medieval music.

Etymology

HEXA- + CHORD(1)