That is to say, the tetrachord b c d e is not written , but : and d e♭ f g is not , but .
The early lyre of the Greeks had four, constituting their tetrachord.
tetrachord, tet′ra-kord, n. a series of four sounds, forming a scale of two tones and a half.
These three classes differ in their arrangement of the tetrachord.
In this case, as the pyknon forms the lower part of a tetrachord, there are two possibilities.
In the enharmonic, the tetrachord consists of two tones and two "dieses."
It naturally disappeared when the tetrachord Hypatn came into use.
But within the tetrachord the reign of diversity was unchecked.
In the diatonic, there are two consecutive tones, and the third interval of a semitone completes the tetrachord.
Thus it appears that it was Didymus, and not Ptolemy, who proposed the tuning of the tetrachord which is now accepted as correct.