In 1992 the Spanish architect Manuel de Solà-Morales already suggested in OASE that the civic, architectural, urban and morphological richness of a contemporary city resides in the collective spaces that are not strictly public or private, but both simultaneously. These are public spaces that are used for private activities, or private spaces that allow for collective use, and they include the whole spectrum in between. Moreover, in the past decades the design of these collective spaces seems to have become an important modus operandi to intervene in the contemporary city. At the intersection between an architectural and an urban scale, architects and urban planners design projects that, through their character and hybridization of privacy and publicity, contribute to the civic, typological and morphological richness of the city. This OASE focuses on the definition, character and role of these hybrid buildings in the urban fabric (or the urban fabric in the hybrid buildings). It raises questions concerning the incorporation, expression and relation of public and collective domains within a building and the relation of building and city. It investigates the relation between the influential role of hybrid buildings in the city and their size and combination of diverse urban functions. It questions the different meanings that are assigned to hybrid buildings. From an architectural point of view, hybrid buildings are often assigned an iconic value or designated as a landmark. From the point of view of urban design, the hybrid building is often seen as a microcosm of the urban condition, replicating the complexity of urban space. From this point of view, the hybrid building is also perceived as an important element of urban policy, being at once an entity that can be mastered and that adds value to city branding — the combination of signs that defines the city.