This volume contains the first evidence of theoretical and medieval music, delivered by two complementary treaties: the first mainly directed to the practice, the second to contemplation. The authors test themselves with a music system still fluid, to organize and regulate. Ubaldo, intended mainly to describe the sound systems of reference and notational aspects, constantly helps the reader with examples drawn from traditions of chant. Regino insert the musical phenomenon in a broader disciplinary framework and creating a sense of history, referring back to the classical theoretical tradition and late-antica. Both authors, though so different, are called upon to perform the same task: to outline the guidelines of an ars that, in turn, needed new rules.