Cities Come in Systems
A city is never alone. It sits in a system of cities, and the system has a shape regular enough to be startling.
The rank-size rule: in a mature urban system, the th largest city has about the population of the largest. Second city, half. Fourth city, a quarter. Tenth, a tenth. It is a rough empirical regularity, not a law — but it holds across wildly different countries and centuries, which is the sort of thing that demands an explanation nobody has fully supplied.
The interesting cases are the violations. When a system does not fit, it is telling you something:
- Primate city: the largest is far bigger than rank-size predicts — often several times the second city. Paris, London, Buenos Aires, Seoul, Bangkok. This is the signature of a system organized around a single centre: a centralized state, an imperial capital, or a colonial economy built to funnel exports through one port. A primate city is usually a political fact wearing a demographic costume.
- Missing middle: the classic colonial pattern. One huge port and lots of villages, with no mid-sized cities — because the economy was built to extract to the coast, and mid-sized cities are what an internal economy grows. What is absent from the distribution testifies as loudly as what is present.
Why Cities Sit Where They Sit
Central place theory (Christaller, 1933) is the classic account, and its logic is simple and powerful. Goods and services have a range (how far someone will travel for one) and a threshold (the minimum population needed to sustain one). A bakery has a small range and small threshold, so bakeries are everywhere. A cardiac surgery unit has a huge range and huge threshold, so there are few, in big cities, far apart.
Stack those and you get a hierarchy: many small centres offering low-order goods, fewer large centres offering everything the small ones do plus high-order goods. On a uniform plain the market areas tile into hexagons, which is where the theory becomes both beautiful and obviously false — no plain is uniform, no transport is isotropic, and rivers exist. But the hierarchy logic survives the caricature, and it explains why the number of cities offering a service falls so predictably as the service gets rarer.
| Model | Claim | Fits |
|---|---|---|
| Concentric zone (Burgess) | Rings outward from the CBD | Industrial Chicago, roughly, in 1925 |
| Sector (Hoyt) | Wedges along transport routes | Cities where a rail line or river dominates |
| Multiple nuclei (Harris–Ullman) | Several centres, not one | Modern car-based cities |
All three are Chicago-shaped, and generalize badly — a European city with a medieval core, or a city whose rich live in the centre rather than the suburbs, breaks Burgess immediately. Use them as vocabulary, not as laws.
The deepest point about cities: they are the densest artefact humans make, and the density is the point. Cities exist because proximity is productive — it lowers the cost of exchange, matching, and imitation, and it always has. This is also why cities have historically been demographic sinks: until sanitation, cities killed more people than they bore and grew only by continuous in-migration from the countryside. A pre-modern city is a place that consumes people, and they came anyway, because it was still the best offer available.