The multiple nuclei model, developed by Ullman and Harris, suggests that growth is independent of the central business district. The sector model, developed by Hoyt, suggests that growth extends along transportation routes.

The concentric zone model, developed by Burgess, describes expansion in concentric rings around the central business district. There are many models of urban structure in the United States.Islamic cities are centered on the principles of the religion. African cities have three separate central business districts (colonial, traditional, market). Latin American cities possess a spine of high-quality housing extending from the central business district. Asian cities are usually built on ports for trade. European cities are older and more historic. Different continents have cities with different characteristics.Other cities are rated and ranked based on their economic, cultural, and political importance to the areas they serve. The three main World Cities are New York City, London, and Tokyo.The largest metropolis in the United States is New York City, with over 18 million people in its metropolitan area. The hierarchy of cities from smallest to largest is hamlet, village, town, city, metropolis, and megalopolis.

Global city definition human geography