he Gregorian chant gives itself to listening through precise 'sound colours': the eight ecclesiastical modes(octoechos). But what led, in the composition of a melody, to the choice of that mode? And how was it interpreted theologically in the Middle Ages? Many hypotheses or pious descriptions can be made, but often they turn out to be only the fruit of the imagination of modern musicologists or theologians. The anonymous medieval authors of this repertoire seem to have chosen certain successions of sounds (mode) not to satisfy a personal taste but with the awareness of using a language that makes use of all the theological loquacity of the first eight numbers to tell a "more": the Mystery. The present volume attempts to open a path of study on the "why" of the choice of the Gregorian mode by presenting the testimony of the liturgical commentary of a French theologian of the 13th century: William of Auxerre and his allegorical doctrine on the mode of Gregorian chant.