 It literally means "four strings", originally in reference to harp-like instruments such as the lyre or the kithara, with the implicit understanding that the four strings produced adjacent (i.e., conjunct) notes.Modern music theory uses the octave as the basic unit for determining tuning, where ancient Greeks used the tetrachord.Ancient Greek theorists recognized that the octave is a fundamental interval, but saw it as built from two tetrachords and a whole tone.