The Size and Distribution of Cities

By the end of this lesson, students will be able to (per CED LO PSO-6.C):

• Identify urban concepts such as hierarchy, interdependence, relative size, and spacing useful for explaining the distribution, size, and interaction of cities (per LO PSO-6.C).
• Define and apply the rank-size rule (per EK PSO-6.C.1).
• Define primate city and identify countries whose urban systems deviate from the rank-size rule (per EK PSO-6.C.1).
• Apply the gravity model to predict interaction between two cities (per EK PSO-6.C.1).
• Explain Christaller's central place theory, including threshold, range, and hexagonal market areas (per EK PSO-6.C.1).
• Use these four principles to predict likely outcomes in urban-system geography scenarios (per Skill 2.C).

Per EK PSO-6.C.1, "principles that are useful for explaining the distribution and size of cities include rank-size rule, the primate city, gravity, and Christaller's central place theory." That single EK is one of the most heavily-tested in Unit 6. All four principles appear in MCQs; multiple appear in FRQs.

Two questions the principles answer

The four principles tackle slightly different questions:

• Rank-size rule + primate city: How large is the second (or fifth, or tenth) city compared to the first?
• Gravity model: How much will two given cities interact with each other?
• Central place theory: Why do we see many small cities, fewer medium cities, and a handful of large cities arranged in a regular spatial pattern?

The simplest principle: in a country with a balanced urban system, the nth-largest city has about 1/n the population of the largest city.

US rank-size: an imperfect but recognizable pattern

The US is the textbook rank-size example. Using 2024 metro populations:

The US has multiple large cities above what the rule predicts, but the overall shape of the distribution is recognizable. No single city dominates.

Why primate cities form

Several drivers usually combine:

• Colonial history: Colonizers concentrated investment in a single coastal administrative capital, leaving the interior underdeveloped. Many Latin American, African, and Southeast Asian primate cities trace to colonial capital formation.
• Centralized government: In countries where most decisions flow through the capital (France, Thailand, Argentina), people and business concentrate there.
• Weak intermediate cities: Without strong regional centers, rural migrants have only one destination.
• Path dependence: Once a primate city forms, its primacy compounds.

Primate vs rank-size as a national signal

You can tell a lot about a country from which model its urban system fits. Federalist countries with diverse regional economies (US, Germany, India, Brazil) tend to approximate rank-size. Centralized countries with colonial histories (France, Mexico, Thailand, Argentina, Peru) tend to have primate cities. The pattern is about political and economic concentration, not just population distribution.

What the gravity model captures

The gravity model formalizes an intuition: big things near each other interact most. It performs well for short-range migration, regional trade, and commuting. It also supports more specific variants (e.g., for air travel, with "distance" replaced by flight time or cost).

Where it breaks down

The gravity model ignores structure. A cultural or linguistic border can sever interaction between otherwise close cities (e.g., the US-Mexico border at Tijuana/San Diego dampens migration-statistics interaction below what the model predicts). Political shocks (Russia-Ukraine war) collapse previously busy city-pair flows. Apply Skill 2.C: the model is useful for predicting likely outcomes under typical conditions but must be adjusted for barriers, policy, and shocks.

Threshold, range, and hierarchy

• Threshold: the minimum population needed to support a good or service. A convenience store has a low threshold (few hundred); a specialty hospital has a high one (hundreds of thousands).
• Range: the maximum distance people will travel to obtain a good or service. A cup of coffee has a short range (< 1 km); a concert has a long range (50+ km).
• Hierarchy: high-threshold/high-range goods are offered in fewer, larger central places; low-threshold/low-range goods are in many small places. A small town has a gas station and a diner; a regional center has those plus a hospital and a mall; a major city has all of those plus a symphony and an international airport.

Why it matters

Christaller's insight explains why rural regions tend toward regularly-spaced small towns rather than random clustering, and why large cities are rarer and more specialized. It is the foundation for retail location decisions, school-district planning, and hospital-network design. It shares DNA with Lesson 5.8's von Thünen model (both build on transport cost) but operates at the scale of city systems, not farm-market distances.

Limitations of Christaller

The model assumes a uniform plain, uniform purchasing power, and rational consumers. Real landscapes have mountains, rivers, and borders. Modern online retail partially collapses the model for low-range goods. But for regional scale understanding (why the Plains states have one small town every 20 miles, for example), central place theory remains remarkably predictive.

Four cases: two rank-size, two primate.

United States · Rank-Size Exemplar

NYC 19.5M; LA 12.7M; Chicago 9.4M

The US has multiple metro areas above 2M and no single primate city. New York is largest, but LA is more than half its size and Chicago is close behind. This federalist, regionally-diverse pattern fits rank-size far better than most countries. It is the go-to AP example for rank-size.

Country page →

Thailand · Textbook Primate City

Bangkok ~11M; 2nd-largest Chiang Mai ~1.2M

Bangkok is ~9x larger than Thailand's second-largest urban area (Chiang Mai). Centralized administration, royal patronage, and concentration of industry and universities combine to produce one of the world's clearest primate cities. AP exam favorite for primate-city identification.

Country page →

Germany · Central Place Theory's Homeland

Multiple mid-sized cities; no primate

Christaller developed central place theory from southern Germany. The country still exhibits a remarkably balanced urban system: Berlin (~3.7M), Hamburg (~1.9M), Munich (~1.5M), Köln (~1.1M), Frankfurt (~770K). Federal structure with sovereign Länder prevented any single city from dominating. Fits both rank-size and central place theory.

Country page →

Argentina · Buenos Aires Primacy

Buenos Aires ~15.5M; Córdoba ~1.5M

Buenos Aires contains about one-third of Argentina's entire population and is roughly 10x the size of Córdoba, the second-largest city. Colonial Spanish administration, European immigration, and centralized federal structure produced this extreme primacy. A classic Latin American primate pattern.

Country page →

1. Per EK PSO-6.C.1, the CED names four principles. Which of the four do you think is most useful for analyzing the US urban system? Which for Thailand's? Defend.
2. If your country's urban system shifted from rank-size to primate, what political or economic changes would have caused the shift? Consider both directions.
3. The gravity model predicts interaction but ignores borders, language, and politics. Identify three city pairs where actual interaction is much lower than the gravity model would predict. Explain the barriers.
4. Christaller's model assumes a flat plain. How might it perform predicting settlement in Japan (mountainous), Bangladesh (rivers), or Chile (long and thin)?
5. Apply Skill 2.C: for a country with one primate city (like France or Thailand), what would likely happen if the government deliberately invested heavily in a second-tier city? What outcomes would you predict?

Draft alignment — pending educator review. AP HG codes correspond to the official College Board Course and Exam Description (Effective Fall 2020, V.1). Statements below are paraphrased in CountryReports' own voice; refer to the College Board's published CED for verbatim wording.

AP® and Advanced Placement® are registered trademarks of the College Board. The College Board was not involved in the production of this material and does not endorse it. Standards statements above are paraphrased; codes refer back to the official College Board CED, the NCSS C3 Framework, the Common Core State Standards, and other cited frameworks.