A general relationship between the size of a settlement and its rank within a set of settlements. If sites are ranked in order of size on a logarithmic scale, the population of the Nth rank city will be 1/Nth the size of the largest; thus the 3rd site will be 1/3 the size of the largest. The rule works best in areas of complex economic and political organization, with comparatively long histories of urban development. It has been suggested that this relationship represents a natural balance of settlement growth. Roman walled towns fit the rule well. However, this is often not the case because in many newer, developing countries the chief city or capital is larger than expected (primate city) because of historic factors.