| In the diatonic, there are two consecutive tones, and the third interval of a semitone completes the tetrachord. |
| In Western music, the tetrachord is an ascending series of four notes. |
| But within the tetrachord the reign of diversity was unchecked. |
| Thus it appears that it was Didymus, and not Ptolemy, who proposed the tuning of the tetrachord which is now accepted as correct. |
| A common instrument from those times was the tetrachord, a kind of four-stringed harp. |
| These three classes differ in their arrangement of the tetrachord. |
