Reproduction can be perceived in two opposite ways from the urban planning point of view, both as a driving force and as a braking weight. Driving force because urban phenomena are precisely produced by re-producing, and especially by reproducing the fundamental relation between public space and private volumes. Strength and longevity of traditional cities usually come from continuities, and peculiarly from this elementary one. The renunciation of this substantial quality is undoubtedly one of the main criticisms addressed to the modern movement. Rediscovered at the end of the 20th century, it could again be underestimated at the dawn of this new century, characterized by reinvention, rupture and search for alternatives. Nowadays, reproduction is widely associated to a deep negative connotation: as an obstacle to personal overtaking, or a resistance to collective innovation. But according to sustainability perspectives, there are probably still lessons to be learned from the urban capacity to cross time with the same intensity. Even to support alternatives, part of continuity can be useful, by imputing historical thickness in vulnerable innovations. While keeping a critical attention, it may be appropriate, even today, to consider reproduction as an urban potentiality. As Bernardo Secchi noted in La ville des riches et la ville des pauvres, urban planning - but also architecture in the largest sense - should take advantage of the capacity of space to oppose the resistance of its own inertia to social change (SECCHI, 2013). He also attributes to space the capacity of proposing an oriented trajectory, linked to this pre-existing form. In this context, the ambiguous trajectory given by the powerful relationship between public and private space deserves to be attentively studied. A careful reading of the evolution of this central dualism reveals the relatively silent presence of a third entity: the collective space. In comparison with the importance of a third way in economic and political fields (OSTROM, 1990), architecture seems to suffer from a significant gap. This transdisciplinary approach brings to light, in the architectural domain, the theoretical discontinuity that has long been given to intermediate space, between public space and private property. The architectural challenge around its own third way has long been reduced to the gradual register, without asserting itself as a status, necessary and complementary to the other two components of urbanity. As a very ancient urban element, and relatively recurrent in the formation of cities, collective space presents both all the transcendent abilities of reproduction, but also a precious flexibility mainly linked to its absence of theoretical formulation. Let's give a new emancipated orientation to this underestimated component, overshadowed by a rooted binary reading of the city.