 A further refinement of tetrachordal construction, according to Aristoxenus, is that the lower interval of the "pyknon" must be smaller than or equal to the upper one .Didymus in the chromatic genus and Archytas in the enharmonic broke this rule, however, and in the "Harmonics" (2.13) Ptolemy criticized this feature in Didymus, holding that it is unmelodic and out of agreement with the evidence of our ears .