a+u 621 is dedicated to the Incidental Space series by Swiss architect Christian Kerez. The result of intense investigations, Incidental Space follows Kerez’s attempts to gauge the potential of ornamental space as a generative device, to seek further liberations and precision in thinking about and defining space. Concepts such as narrative space, fluid space, and atomized space are anthologized here in one of Kerez’s two essays. In the other essay, Kerez documents and reflects on his experiences in Baroque architect Francesco Borromini’s canonical buildings, translating them “from the medium of architecture into the medium of language.” Unusually for a+u, only 6 projects, 3 built and 3 unbuilt, are featured in this issue. Sketches, drawings, and models exhaustively archive the design process of each project, from the testing of concepts to the final form and spatial experience. Projects are never linear in Kerez’s disciplinary interrogation of architecture but rather take the form of research and experimentations. Together these projects underscore how spaces can be created – not merely found, but made with rigor – in the caprices of incident.