The linguistic situation of the Medieval West has sometimes been characterized as one of diglossia: one learned language was used for religion, law and documents - Latin, the father tongue - while the various vernaculars would have been the mother tongue. In Romance-speaking areas, however, the relevance of the term diglossia has been contested, and the date of the divergence between written or spoken Latin and Romance is a subject of energetic debate. How can one characterize the interaction between Latin and the various vernaculars? To what extent could speakers from separate linguistic worlds communicate? These questions are fundamental for anyone concerned with almost any aspect of communication, the transmission of learning, literary history, and cultural interaction in the Middle Ages. To approach the question in its broadest context, one must consider many categories of evidence: glossaries, colloquies, glossed texts and vernacular translations, grammars, and anecdotes in narrative sources about the study of foreign languages. The background to the relat