Where one city towers over all the others, the country has a primate city instead.

The arithmetic behind the rule

The rule takes the largest city's population and divides it by each city's rank. If the largest city holds 8 million people, the rule predicts 4 million for the second, about 2.7 million for the third and 800,000 for the tenth. Plotted on a graph with logarithmic axes, those points fall on a straight line sloping down. The pattern is a version of Zipf’s law, which describes the same one over n drop in word frequencies and many other ranked lists.

What a rank size pattern says about a country

A country whose cities follow the rule usually has cities of every size, each serving its own region. Jobs, universities and government offices are spread out, and goods move between many urban centers. The United States is the standard exam case: New York, Los Angeles and Chicago form a rough ladder of large metropolitan areas rather than one giant. Geographers read this rank size distribution as a sign of an integrated national economy with no single dominant center.

How a primate city breaks the pattern

A primate city is far larger than the second city and dominates the nation's economy, politics and culture. The geographer Mark Jefferson named the idea in 1939 and described a primate city as at least twice as large as the next. Under the rank size rule the first city is already double the second, so the clearer exam signal is a gap many times wider. Bangkok in Thailand, Mexico City and Paris are cases students can name.

Why some countries grow one giant city

Primacy usually follows concentrated power. In many former colonies the colonial capital became the port, the seat of government and the main market at once, and later investment kept flowing there. Migrants follow the jobs, which widens the gap. In a centralized state, decisions made in one city pull banks, media and universities toward it. The course's own scoring guidelines add a political risk. A primate city surrounded by neglected regions can create pressure for devolution.

Two slips the exam punishes

The first slip calls any largest city a primate city. Every country has a largest city; primacy requires a size gap far beyond the rule's prediction and dominance in national life. The second slip treats the rule as exact. Real cities only approximate it, so judge the pattern from the ratios, not from a perfect match. Keep the rule apart from central place theory as well: one predicts population from rank, the other explains how far apart places sit.

Reading a population table on the exam

The course framework includes a sample question that lists the largest cities of two countries and asks which pattern each follows. Divide the first city by the second. A result near two fits the rule, and a result far above two signals primacy. Then check the third city against a third of the first. In a free response answer, name the pattern and give one cause. Drill the ratios with a multiple choice test maker, revisit them through spaced repetition, and review the Unit 6 page.

What is the rank size rule in AP Human Geography?

A model stating that the nth largest city in a country has about 1/n the population of the largest. The second city holds about half as many people, the third about a third.

What is a primate city?

A city far larger than the second city in its country that dominates national economic, political and cultural life. Mark Jefferson named the idea in 1939. Bangkok in Thailand is a standard example.

Does the United States follow the rank size rule?

Roughly, yes. It has a ladder of large metropolitan areas, led by New York, Los Angeles and Chicago, instead of one giant city, which is why textbooks use it as the standard example.

Why do some countries have primate cities?

Power and investment concentrated in one place. A former colonial capital or the seat of a centralized government gathers jobs, ports and services, and internal migrants keep moving there, widening the gap.

How do I check whether a country follows the rank size rule?

Divide the largest city's population by each city's rank and compare the result with the real figures. If the first city is many times larger than the second, the country shows primacy instead.

How is the rank size rule different from central place theory?

The rank size rule describes city populations statistically by rank. Central place theory explains the spacing of settlements and the services each level offers, using threshold and range.

Last updated: 2026-10-03 · Written with AI assistance and reviewed before publishing.

The rank size rule predicts a city's population from its place in line