The paper focuses on the notion of old venetan (language or dialect), in the frame of a research project about the Grammar of the medieval dialects of the Veneto area based on the TLIO corpus (Tesoro della lingua italiana delle origini, Cnr). It argues that (and explanes why) it is basically correct to consider the medieval vernaculars of this area as a typical diasystem, with some specific characters and features that must be taken into account. A critical assessment of the chronological limits chosen for such a linguistic analysis is proposed, by sketching an overview of the geography and history of the Romance vernaculars of this region in late Middle Ages.