The introduction of the natural duple rhythm into music written for the church demanded the addition of new signs to the mensural system of notation (cf. Chap. VI, pp. 177 ff.), for it was necessary that singers should be informed whether they were to sing according to the triple or double scheme. Thus there appear about this period new time signs. Of these a semi-breve, still called, by the way, _tempus perfectum_ circle, [music sign], signified the division of the breve into three or perfect time. A half-circle, [music sign], signified the division of the breve into two semi-breves, and this was imperfect time. A dot within the circle or the half-circle, [music sign] [music sign], indicated that the semi-breve was to be divided into three minims, but without the dot the semi-breve equalled only two minims. The three-part division of the semi-breve constituted major prolation, the two-part, minor prolation. Perfect or imperfect time was sung twice as fast if the time sign was cut by a line, [music sign] [music sign]. The second of these cut signs still survives in the modern sign, [music sign], signifying _alla breve_ time. It appears likely that De Vitry himself was the first to think of using colored notes to signify still another genus of rhythmical subdivision called _proportio hemiolia_; and that he was the first to use the term _contrapunctus_, or counterpoint, instead of descant. Through lack of actual examples of the period we are unable to tell how thoroughly and readily church composers adopted the methods of the _ars nova_, but eventually their advocacy was of momentous importance. It is true that secular music was the first to benefit by the advance, for it preserved naturally all the elements which the new law purposed to regulate. Hence the first form--that which constitutes the first ground of interaction, the transition to the polyphonic form of church music--was the popular _chanson_, an elementary form of song, evidently developed from the _canson_ and the ballad of the Troubadours, etc., which, as we know, were composed for a solo voice with an improvised instrumental accompaniment. According to Riemann this development of the chanson first went forward in Italy, in connection with the movement known as the Florentine ars nova, a detailed account of which we have chosen to reserve for our next chapter. The Italian ars nova, which is held by modern historians to have influenced the French ars nova in various ways, and to have transmitted to it a style of composition in which the upper voice was freely invented and harmonically interpreted--though in a rude manner--by the accompanying voice or voices, a style which by 1400 was fully developed. These chansons were, it should be noted, like their prototype, chiefly for one solo voice with instrumental accompaniment and varied by instrumental preludes, interludes, and postludes. Purely vocal polyphony in chansons was rare before 1500, though examples