Urban Geography and City Systems

Cities Come in Systems

A city is never alone. It sits in a system of cities with a shape regular enough to be startling.

The rank-size rule states that in a mature urban system, the nth largest city has about $1/n$ the population of the largest. The formula is $P_n \approx P_1 / n$. This empirical regularity holds across wildly different countries and centuries.

The interesting cases are violations. A primate city is far bigger than rank-size predicts, often several times the second city, as seen in Paris, London, or Seoul. This is the signature of a centralized state, imperial capital, or colonial economy funneling exports through one port.

The missing middle is the classic colonial pattern of one huge port and tiny villages with no mid-sized cities. It reveals an economy built for extraction rather than an internal market.

Why Cities Sit Where They Sit

Central place theory (Christaller, 1933) explains city spacing using two concepts. Range is how far someone will travel for a good, and threshold is the minimum population needed to sustain it. Bakeries have small ranges and thresholds; cardiac surgery units have huge ones, creating a hierarchy.

Cities exist because proximity is productive, lowering the cost of exchange and imitation. Historically, cities were demographic sinks that grew only by in-migration from the countryside.