The central idea of this chapter is koineisation, the process by which discrete varieties tend to form into a new compromise variety when speakers of these varieties find themelves living side by side. Dialect levelling and new dialect formation are central forces in the process. The case study considers what happens when closely related but discrete language varieties come together in new circumstances. Primary focus is given to contact between Scandinavian varieties and Low German in the late medieval and early modern periods and between Old English and Viking Norse in northern England in the early medieval period.