This article examines the preservation of medieval books in Reformed libraries in the territory of present-day Switzerland. Given the persistence of late medieval practices and approaches in Protestant academia, it examines whether the example of libraries reveals a kind of institutional inertia, i.e. whether the pressure of existing collections and the normativity of de facto established practices gave the medieval library heritage a weight that slowed down radical change and disruption. It is shown that the medieval legacy did indeed exert a kind of formal pressure on the design of reformed libraries, which can even be traced down to the formulation of doctrine. However, the article also argues that the resulting continuities are by no means negative, but in some cases led to highly original approaches.