hex·a·chord 
(hĕksə-kôrd′)
(hĕksə-kôrd′)n.
A sequence of six tones with a semitone in the middle, the others being whole tones, that was used in medieval music.
[Medieval Latin hexachordum, from Latin hexachordos, having six strings or stops : Greek hexa-, hexa- + Greek -khordos, string, note (from khordē; see CORD).]