[64] This dreaded interval was called by churchmen _diabolus in musica_, and as such studiously avoided. CHAPTER VI THE BEGINNINGS OF POLYPHONY The third dimension in music--'Antiphony’ and Polyphony; magadizing; organum and diaphony, parallel, oblique--Guido d’Arezzo and his reputed inventions; solmisation; progress of notation--Johannes Cotto and the _Ad organum faciendum_; contrary motion and the beginning of true polyphony--Measured music; mensural notation--_Faux-bourdon_, _gymel_; forms of mensural composition. In the preceding chapter we have tried to trace the perfecting of a form of melody called plain-song. We have seen how the mass of the Catholic Church was set to solo music. Apart from the highly expressive quality which the music inevitably acquired because of the reality and life of the new emotional religion, the plain-song of the mass did not differ from the artistic music of the Greeks and the Romans, that is to say, it brought forward no new means of effect or of expression. We may say it was the adaptation of old and tried methods to new ends. We can hardly suppose that the technique of composition had been advanced by the early Christian composers beyond the point to which the Greeks had brought it, nor that the art of music had been expanded during the first centuries of the Christian era to greater proportions than the Greeks had developed it. The theorists of the first nine centuries made blunders in trying to systematize Christian song according to the remnants of Greek theory which had been preserved; yet the Greek scales were still in use, though misnamed by the theorists, and composers for the church still conformed to them. But about the beginning of the ninth century a new element appeared in music for the church which the Greeks had left practically untouched and which was probably the contribution of the barbarian peoples of northern and western Europe, either the Germans or the Celts, namely, part-singing. To the single plain-song melodies of the ritual composers added another accompanying melody or part. The resultant progression of concords and discords was incipient harmony, the practice of so weaving two and later three and four melodies together was the beginning of the science or art of polyphony. I