Line up a country's cities from largest to smallest. Is there a pattern in their sizes? In many countries there is, and its exceptions tell a story too. This builds on central place theory and uses division as in fractions as division.
Key idea
The rank-size rule says the city of rank $n$ has about $1/n$ the population of the largest city: the 2nd largest about half, the 3rd about a third, and so on. A primate city is more than twice as large as the next largest city, and dominates its country.
Why it works
The rule is an observed pattern (related to a broader one called Zipf's law), not a law of nature. It shows up where a large country has a full hierarchy of central places. A primate city usually reflects history: the capital of a once-centralized kingdom or a colony, where government, trade, ports and universities all gathered in one place. The geographer Mark Jefferson named the pattern in 1939; his examples included Paris and Bangkok.
Worked example: Predict a city
The largest city in a country has 6,000,000 people. If the rank-size rule holds, about how many live in the 4th largest city?
- Rank 4 should have about $1/4$ of the largest city.
- $6{,}000{,}000 \div 4 = 1{,}500{,}000$.
Answer: About 1,500,000.
Worked example: Primate or not?
Country X's largest city has 8 million people and its second has 1.5 million. Is the largest a primate city?
- Primate means more than twice the second city.
- Twice the second: $2 \times 1.5 = 3$ million.
- 8 million is far more than 3 million.
Answer: Yes. It is more than five times the second city.
Common mistake
Thinking every country follows the rank-size rule. Countries with a primate city, such as the United Kingdom (London) or Thailand (Bangkok), break it. Small countries and countries with a long history of one central capital often have primate cities.