The recent theory of planning and urban design highlights how healthy, vibrant cities or neighbourhoods behave as open systems (Sennett 2013; Christiaanse et al. 2009), whose main characteristic is the ability to incorporate change and adaptation and to permit the emergence of unpredictable social and environmental situations. Sennett (2013) defines this kind of urban environment as an ‘‘open city’’: a city which is ‘‘incomplete, errant, conflictual, nonlinear’’. He highlights the contrast between open and closed systems: an urban system is open when it emerges from unexpected interactions between physical creation and social behaviour, when it allows ‘‘jerry-built adaptations or additions to existing buildings’’. Instead it tends to become closed when a regime of power of any kind, which wants order and control, aims to over-determine both the city’s visual forms and its social function. Rieniets (2009, p. 13) associates the concept of the open city with the word ‘‘coexistence’’, highlighting how a city behaves as an open system when it allows the unprogrammed congregation and encounter of different people and ‘‘provides all of its inhabitants access to the concentrated multitude of opportunities they have at their disposal’’. Christiaanse (2009) stresses how through the concentration and interaction of people from different backgrounds, the open city stimulates economic growth, innovation and cultural emancipation. All these authors highlight the need to apply the concept of the open city to urban design practice (as well as the difficulties in doing so). Christiansee (2009, p. 31) stresses how the open city ‘‘is not a clear-cut urban vision, but a volatile situation, a precarious balance between the forces of integration and disintegration’’. Sennett emphasises the fact that every attempt to over-determine a city’s visual form contrasts with its non-linear unpredictable nature and tends instead to produce a closed system. The contrast between top-down planning and the unpredictable emergent nature of cities was already highlighted by Jane Jacobs (1961, p. 14), who observed that city vitality was ‘‘something that the planner or planners alone, and the design and designers alone, can never achieve’’.