In the "_Epistola de Harmonica Institutione ad Rathbodum Episcopum Trevinesem_" ("Letter to Rathbodum, Bishop of Treves"), there is mention of the instruments of music during the seventh and eighth centuries. They are the cithara and harp as the stringed instruments; musetts, syrinx and organ among the wind instruments; cymbals and drums, instruments of percussion. In the tenth century there was a methodical treatise upon music in dialogue form, published by Odon, abbot of Cluny, who died in this monastery November 18, 942. This work, which was wrongfully attributed to Guido of Arezzo, contains a number of analyses of intervals showing an understanding of the exact dimensions of the various kinds of fourths, fifths, thirds and sixths. According to his doctrine, the intervals of the fourths, fifths and octaves are more natural for the voice than the others called thirds and sixths, because the former are invariable, while the latter may be larger or smaller by a half step. He makes a summary of ecclesiastical chant, mentioning the modes as established by St. Gregory, illustrating each of them by a selection from the "Plain Song." It is a fact significant of the unsettled condition of musical theory and the complete unconsciousness of musical amateurs that any essential change in the art was being undergone, that as late as 1000 or 1020 Adelbold, Bishop of Utrecht, published a treatise upon music in which the proportions of the tetrachords are calculated carefully according to the Greek theories, and demonstrated upon the monochord. III.