The ground floors of buildings are a key element of the urban experience, yet the dynamics that shape frontages are largely unknown. This article delves into the forces and patterns behind the transforming relationship between architecture and public space in Western urban cores over the past century. After defining a methodology for structurally measuring the interactivity of ground floor frontages over time, the study focuses on two case study urban cores of Detroit, Michigan and The Hague, Netherlands. Through a combination of narra- tive historiography, detailed mapping and statistical studies a set of recommendations is generated to help urban designers and planners better understand and counter frontage decline. The two seemingly disparate cities are demonstrated to have undergone remarkably similar patterns of frontage interactivity erosion, with outcomes diverging as a result of an often reinforcing set of forces. Only upon understanding frontages as social, economic, cultural, political and technological constructs with physical, functional and connotative effects on public space will the profession be able to effectively steer the future of the architecture of public life.