It is proposed that our understanding of medieval town foundation is limited by a failure to appreciate that ‘town’ is a relational category. It is argued that urban character emerges from social relations, with some sets of social relationship revealing urbanity and others not, as places develop along distinctive, but related, trajectories. This argument is developed through the application of assemblage theory to the development of towns in thirteenth-century southern England. The outcome is a proposal that, by focusing on the social relations through which towns are revealed as a distinctive category of place, we can better comprehend why and how towns mattered in medieval society and develop a greater understanding of the relationship of urbanization to other social processes such as commercialization and associated changes in the countryside.