Note 14 — Chapter VII · Scales

The word tetrachord means literally "four strings" and refers to the primitive instrument, the four strings of which were so tuned that the lowest and the highest tones produced were a perfect fourth apart. With the Greeks the tetrachord was the unit of analysis as the octave is with us to-day, and all Greek scales are capable of division into two tetrachords, the arrangement of the intervals between the tones in each tetrachord differentiating one scale from another, but the tetrachords themselves always consisting of groups of four tones, the highest being a perfect fourth above the lowest.