The new diocesan breviaries and missals adopted in France during the reigns of Louis XIV and Louis XV came with an important reform of plainsong. Manuscripts, local vocal traditions, but also aesthetic conventions and pastoral requirements of the time had to be taken into account by the 18th-century plainchantistes when (re)composing a repertory. The Traité of Léonard Poisson, who had been at the origin of the chant in Sens and Auxerre, was written in reaction to defaults observed in the immense production of plainsong already published. However belated, his contribution throws an unrivalled look on the overflowing activity of liturgists. With a rare precision, Poisson analyses their aims as their methods, their defaults as their qualities. Far from the ethereal and spontaneous creation of Gregorian historiography, it is the discovery of a real process of composing still misunderstood that the study of this Traité unveils.