Abstract This article discusses the transition from a system with contrastive, segmental quantity in Old Norse into the present day system characterizing most Norwegian and Swedish dialects, where stressed syllables are obligatorily bimoraic. Starting with variation within East Norwegian, two intermediate varieties between Old Norse and the modern system are identified, and the four varieties are then related to each other by means of constraint reranking within an Optimality Theory analysis. A full factorial typology based on the four constraints involved is then developed. This renders four possible intermediate stages between Old Norse and the modern system, of which two are attested in East Norwegian. When the scope subsequently is widened to all varieties of Norwegian and Swedish, it is shown that all the intermediate varieties predicted by the analysis are attested. More importantly, no other varieties than those predicted seem to exist, even if such varieties can be construed. This suggests that the grammar developed to account for the changes not only is empirically adequate, but also has explanatory value.