My dissertation is an investigation of the two masterpieces of medieval, allegorical literature from the perspective of the Latin moral tradition of their time. Discussing Jean de Meun and Dante's obsessive concern with the sinfulness of speech, I relate the numerous verbal transgressions treated in the Romance of the Rose and the Divine Comedy to what historians of moral philosophy have called "the golden age of the sins of the tongue" (1190-1260), a time span during which moralists, theologians and canonists wrote a great number of Latin texts on peccata linguae. I argue that the radical inclusion of the sins of speech among the other classes of sins treated in the Romance of the Rose and the Divine Comedy is to be accounted for in light of the major thirteenth-century treatises on peccata linguae. While Jean de Meun, in the wake of Alain of Lille, treats the sins of the tongue in a dispersed manner, without regard to a classification based on the gravity of the sins, Dante follows a scholastic approach and assigns most of the sins of tongue he is dealing with to the infernal area of Fraud, in a hierarchical order. Taking up elements from William Peraldus's Summa vitiorum and Thomas Aquinas's Summa Theologica, both very popular at the time, Dante constructs his own micro-system of peccata linguae, a system within a system. Written shortly after the golden age of the sins of the tongue, the Romance of the Rose and the Divine Comedy extend this cultural period and transfer the preoccupation with sinfulness of human speech from the exclusive sphere of Latin moral tracts to the realm of vernacular poetry.