The proposed principle for understanding the urban fabric is based on considering the town as a set of complex objects, taking a systemic approach. The town system used to study the urban fabric over large time spans is composed of three sub-systems relating to historical objects from the level of the excavation to that of the former urban space: function (social use), space (location, surface area and morphology) and time (dating, duration and chronology). The historical object is the analytical unit of the space studied. It is the Cartesian product of the three sets, Social use, Space and Time, from which it stems.On the basis of this process, the Historical Object (OH) is broken down into three types of simple object, functional (EF), spatial (ES) and temporal (ET).- The thematic approach to the OH in an urban environment is based on social use, organized according to a hierarchical thesaurus.- Space, the most formalized of the three sets, is structured on the model of a planar topological graph without isthmi.- Time, always considered as continuous and linear, will be modelled by analogy with space using temporal topology defined in the field of artificial intelligence.The relationships between these three sets each characterize an interaction (social use-space, social use-time, time-space, or function-space-time). In addition to reconstructing the OH, they allow urban changes to be observed by analyzing the distributions and mapping of each of the entities singly or two-by-two. The originality of this procedure lies in its approach whereby it is possible to start not from the mapping of a phenomenon at a time t1 and comparing it to that at a time t2, but to look at it in the same way whether its input is social use, space or time. The heuristic value of this modelling lies in the shift from description (what, where, when) to understanding the phenomena of change (how, why).