In medieval Europe, for example, there was a clear-cut boundary between urban and rural, usually in the form of a wall. Subsequently, towns have progressively overflowed their municipal boundaries. In the 1960s, UNO introduced the concept of the urban agglomeration (UK—urban area; France—unité, Germany—Verdichtungsraum) in order to harmonize the statistical definitions of urban settlements from one country to the next. The extensive suburban sprawling based on mass car ownership led to the concepts of daily urban systems, or Standard Metropolitan Statistical Areas (SMSAs). Later, the French statistical system defined the aire urbaine: a contiguous urban core and its commuter belt. About 360 aires urbaines were defined (Le Jeannic (1996) Cybergeo écon. & stat. 294/295).