The central idea behind the policy concept of urban networks is that the total of a metropolitan area is more than just the sum of its component cities. This is expressed in the idea that cities can complement each other and that they have a larger 'mass' together, resulting in a larger support for certain facilities. Unfortunately, much of the potential for synergy between cities in an urban network remains untouched. That is the conclusion of Evert Meijers in his dissertation on synergy in polycentric metropolitan regions, including the Randstad and the German Rhine-Ruhr area. In his research, he illuminates three aspects that could provide synergy among cities.