Only two treatises upon music have come down to us from the ninth century. The first is by a monk, named Aurelian, in the abbey of Reome or Montier-Saint-Jean, in the diocese of Langes, who appears to have lived about the year 850. His book, called "_Musicae Disciplina_," in twenty chapters, is a compilation of older anecdotes and theories, throwing no light upon the actual condition of the art in his day. The sole remaining work of this period was by Remi, of Auxerre, who had opened the course of theology and music at Rheims in 893, and afterward at Paris in the earlier years of the tenth century. His book, like the preceding, is wholly devoted to the ideas of the ancients. II. This brings us to the first writer on music, during the Middle Ages, whose work throws any important light upon the actual practice of the art in the period when it was written, namely, Hucbald, a monk of the convent of St. Armand, in the diocese of Tournay, in French Flanders. Gerbert gives two treatises upon music, as having come down to us from this author. Nevertheless there is reason to doubt the genuineness of one of them--whereof presently. The first of these, the so-called "Treatise," from a manuscript in the library of the Franciscan convent at Strassburg, collated with another from Cesene, bears this title: "_Incipit Liber Ubaldi Peritissimi Musici de Harmonica Institutione_." The other is called "_Hucbaldi Monachi Elonensis Musica Enchiriadis_," or "Manual of Music, by the Monk Hucbald." The former work is of little interest, and if a genuine production of Hucbald's, probably belongs, as M. Fetis suggests, to his earlier period, when he was still teaching at Rheims, along with his former classmate, Remi, of Auxerre. The manual of Hucbald is not to be regarded as a complete treatise upon music. It has three principal subjects, namely: The formation of a new system of notation, the tonality of plain song, and symphony, or the singing of many voices at different intervals--in other words, harmony. In treating the scale he divides it into tetrachords, precisely according to the Greek method, as far as known to him, and he nowhere appears to perceive the inapplicability of this division to the ecclesiastical modes. For representing the sounds of the scale, divided into four tetrachords, Hucbald proposed the Greek letters, which in effect, would have been a notation of absolute pitch, with the farther disadvantage of ignoring the harmonic principles of unity already discovered, and in fact involved in his own method of enlarging a two-voice passage by adding a third at the interval of an octave with the lowest. He recognizes six kinds of symphony; in reality he employs only three, the others being reduplications. His symphonies are those of fourths, fifths and octaves. In all parts of his work but one he uses the term diaphony as synonymous with symphony; _there_ he gives its ancient meaning of dissonance.