In 1305, shortly after his Vita nova, Dante wrote his De vulgari eloquentia, a text which served as a `manifesto' for the champions of the Italian vernaculars and for the defenders of the European vulgar tongues which were rapidly expanding at the end of the 14th century. Dante suggested that poets wanting to `illustrate' the mother tongue should compose a work as magnificent as that of his great Roman master Virgil, a work not defined generically but by its eleven-syllable metre and its `illustrious' subject: salus, virtus, venus. Boccaccio, the first of Dante's disciples, responded to Dante's call by writing a work dedicated to Mars, the all-powerful warrior god who alone could grant eternal glory to poor mortals. Only a few years after it appeared, the Teseida della nozze d'Emilia served as a model for English and French poets. Chaucer translated the Teseida, which became "The Knight's Tale," and, around 1380, included it in his Canterbury Tales. In France, an anonymous translator at the court of King Rene of Anjou translated Boccaccio's text around 1460, giving it the title Thezeo. In the 16th century, a period of fundamental importance in the history of the French language, Anne de Graville translated the Thezeo into more modern French and gave it the title: Le Beau romant des deux amants Palamon et Arcita et de la belle et saige Emilia. Our study follows Boccaccio's Teseida from one language to another, taking particular interest in Dante's project, and in the `counter movement,' to illustrate the European vernaculars.