In the antechamber of Pope Eugene IV in 1435 there took place a debate among a group of Apostolic Secretaries which was to continue to excite the interest of leading Italian humanists for many decades there after and even into the 16th century. A number of these Secretaries, among them Biondo Flavio, argued that in ancient Rome literate and illiterate spoke a Latin which, though obviously differing in lexical, syntactic and stylistic complexity, was basically one and the same language. The spokesman for the others, Leonardo Bruni, maintained that the vulgus must have used a vulgaris sermo as sharply distinct from the latina ac litterata lingua as the vernacular of their own times was from Latin. As has rightly been pointed out, the disagreement between these humanists was more nominal than real in that the arguments put forward derive directly from diverging notions of ‘language’ and ‘grammar’: hence the two sides evaluate diglossia in the 15th-century society and, therefore, the situation in ancient Rome, by different criteria. This article (which summarizes a forthcoming book), traces the sources for and analyses the terminologies employed in the various texts in an attempt to make their true meaning plain and also uncover the underlying reasons for mutual misunderstanding. It also points out the major shifts of focus which occurred in the subsequent controversy. While Leon Battista Alberti exploited the discussion for a pro-volgare campaign of his own, Guarino Guarini’s exploration of the term litteralis represents one of the most sophisticated points of view adopted by a grammarian, and the polemic between Poggio Bracciolini and Lorenzo Valla over latine and grammatice loqui reflects a division within the humanist movement of more far-reaching implications.