One of the paradoxes of Gregorian chant is the way in which written sources become ever more plentiful across the Middle Ages while commentaries on its cultural and intellectual status take the opposite direction, becoming rare after the ninth century. An exception to that trend is the essay De varia psalmorum atque cantuum modulatione (On the Varied Modulation of Psalms and Chants), a substantial yet little known offering from the music theorist and liturgist Berno of Reichenau (d. 1048). Previously considered to be of uncertain authorship and doubtful musical value, the work is now shown to be an authentic witness, in part through evidence provided by a rediscovered manuscript (Berlin, Staatsbibliothek, Mus.ms.theor. 95). This permits a new appreciation of the author's unique and revealing agenda—to soothe the many tensions reportedly incited by the textual content of chant. With resonances in contemporary music theory, De varia psalmorum testifies to divergent practices in need of a new theoretical underpinning, as well as to previously unstudied cultures of textual correction existing between the ninth and twelfth centuries. In so doing it offers a rare insight into the liturgical chant traditions of the post-Carolingian age, both in Berno's native Germany and further afield.