Before the late eighth century, with a few exceptions (epigraphy, the languages of the Caucasus and the North-Western Isles), little had been written in Europe in languages other than Latin, Greek and Hebrew. The long ninth century saw this monopoly of the ‘three sacred languages’ shaken and challenged: several vernacular languages (Celtic, Germanic and Slavonic, but also, if to a lesser extent, Romance) appeared in writing for the first time and others developed in significant ways. This paper, introducing this special issue on Vernacular Languages in the Long Ninth Century, starts with an assessment of the linguistic situation of Europe before the changes began; it goes on with a summary of the main developments known to have taken place in the long ninth century; it then addresses their possible connections and observable entanglements and identifies the conditions that allowed the emergence and the sustained flourishing of written vernaculars.