Building the contemporary open city

With Modern Movement experience the way of building the city starts to change. The close relationship block-street-houses progressively overcomes the traditional urban composition because of the changed size of the dwelling spaces. In the city's design composition is introduced a topological inversion, different from the ancient city, due to the density relationship transformation between mass and void resulting from the urban expansion. The principle of openness, since then, has thus become the hinge building principle of the city replacing the delimitation one in order to conform a block that could be defined as open where what is important is the - not random - arrangement of the buildings in the space. The concept of openness could be declined in different ways and can be referred to the “externality”concept introduced by Uwe Schröder. The German architect through the Pardiè project has introduced a particular way of reading the urban space in which the unbuilt space can take different characterisations as a field or as an externality according to the limit conditions. The essay analyze two urban compositions in Milan considered as exemplar by re-reading these dual possibilities of understanding openness as a valid way of building the city of today