Francoprovençal has generally been viewed as an oral dialect group with a highly varied character (due to the mountainous regions of the Alps, the Jura and the Massif Central), and with no elaborated written textual tradition. The virtual absence of such a tradition may indeed be observed for the modern period (i.e. the second half of the second millennium). However, this is not the case for the medieval period, during which Francoprovençal underwent a process of elaboration similar to that of the neighbouring Romance languages, at first fragmentary and embedded in Latin. From these beginnings emerged a pure Romance <jats:italic>scripta</jats:italic> which remained in existence until the end of the 15th century, when it was replaced by French. This process remained incomplete, characterised by the continuous copresence of either Latin or French and essentially limited to legal and administrative documents. Nonetheless, the first half of the second millennium yields a complex written vernacular which displays both diatopic and diachronic variation, and which has never before undergone systematic analysis. The present contribution is based on a corpus of medieval documents from the Francoprovençal regions of France and Switzerland (approx. 700.000 words), digitised as part of the project <jats:italic>Documents linguistiques galloromans</jats:italic> and presented for the first time here. Taking one of the most salient and frequent phenomena as an example – the outcome of lat. /a/ in tonic free syllables (<jats:sc>ˈpratu</jats:sc>> Frpr. <jats:italic>pra</jats:italic> vs Fr. <jats:italic>pré</jats:italic>) – the authors assess the nature and extent of the presence of Francoprovençal in medieval scripturality, examining a total of 60.000 occurrences. The results thus obtained allow an interpretation of the historical development of Francoprovençal on the basis of entirely new evidence.