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Power laws (and the special case of Zipf’s law) allow us to characterize cities on a map with a single number, namely the slope of a rank-size curve. These rank-size curves show the rank of a city as a function of its size: the biggest city gets rank 1, the second largest city rank 2, and so on. The slope tells us whether cities are relatively similar in size (small slope) or unevenly sized (steep slope). But what explains the existence of different sized cities? This chapter introduces some urban theories that explain the existence of different sized cities; a graphical representation of the Henderson model and a graphical representation of a state-of-the-art model of differentiated cities as developed by Davis and Dingle.