This art, again, is clearly due to the influence of the round singing of the British isles. Thus we have already a beginning of at least three important elements of good music: The recognition of the triad, or, more properly, of the third and sixth, a beginning in imitation, and the contrapuntal concept of an independently moving melodic accompaniment to a second voice, which in turn had been the outcome of extemporaneous descant. The works of Perotin were undoubtedly in advance of his time, having in them no small vitality, as is shown in their having formed a part of the repertory of Notre Dame for more than two centuries. The second period of the old French school extended from about 1140 to 1170, and great improvements were made in the art of harmony meanwhile. The three great masters of this period were Robert of Sabillon, his successor in Notre Dame, Pierre de la Croix, and a theoretical writer named Jean de Garland. The first of these men was distinguished as a great deschanteur, in other words, a ready hand at extemporaneous counterpoint. Pierre de la Croix made certain improvements in notation, the nature of which, however, the musical historians fail to give us. Garland divided the consonances into perfect, imperfect and middle--a system which has remained in use, with slight alteration, to the present day. The thirds and sixths, however, still rank as dissonances. He also defines double counterpoint, and gives examples. The illustrations are crude, but the idea is correct. The third period of the old French school is sometimes known as the Franconian period, from the two great names in it of Franco of Paris and Franco of Cologne, whose theories have already been noticed. (See page 146.) Another celebrated name of this period was that of Jerome of Moravia, also a theoretical writer, whose treatise has been published along with the others in Coussemaker's "Mediaeval Writers upon Music." He was a teacher and a Dominican monk at Paris. He was contemporaneous with Franco of Cologne.