The chants of the latin Church were transmitted orally from the beginning until the ninth century. A number of variants, which can be spotted in the manuscripts which were preserved from the Carolingian ages, show the diversity of coexistant traditions in the medieval Europe, even before the invention of musical notations. The study of tonal, modal or ornamental variants, whether they are intentional or unintentional, invite to distinguish between errors and authentic choices made by the copist, who is a real witness of the local and cultural traditions. The intellectual or musical interpretations of these chants will take advantage of this rich diversity, much more than the tendency to impose a uniformity, which has no roots in History.