Origin of tetrachord
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Thus it appears that it was Didymus, and not Ptolemy, who proposed the tuning of the tetrachord which is now accepted as correct.A Popular History of the Art of Music|W. S. B. Mathews
In ancient language, the tetrachord Synmmenn (a-b♭-c-d) takes the place of the tetrachord Diezeugmenn.
In this case, as the pyknon forms the lower part of a tetrachord, there are two possibilities.
Along with this change we have to note the comparative disuse of the Enharmonic and Chromatic divisions of the tetrachord.
In practice it appears that musicians tuned the tetrachord b-e of this scale with the Pythagorean two Major tones and leimma.