Learn · AP® Human Geography

Unit 6.6 — Synthesis: Comparing and Applying the City Structure Models

This unit closer lines up Burgess, Hoyt, and Harris/Ullman side by side, adds the regional variants that extend them, and walks through how to argue a model choice with evidence.

17 minUnit 6AP® Human Geography
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This closing lesson pulls the unit's separate pieces back together. You have now seen why cities grow (urbanization and its worldwide patterns), how they are organized internally (Burgess, Hoyt, and Harris/Ullman), how they rank against each other (central place theory, rank-size versus primacy), what strains they generate as they grow (sprawl, gentrification, food deserts, housing cost), and how a small set of them dominate the global economy (world cities and globalization). The synthesis skill this unit is really building toward — and the skill AP Human Geography's free-response section rewards most directly — is the ability to compare the three internal-structure models against each other and apply the right one, with real supporting evidence, to an actual named city.

The three models side by side

Line the three models up on the same set of questions and their differences become sharper than studying each one in isolation ever shows. Burgess's concentric zone model (1923) says land use sorts into rings expanding outward from a single CBD, with the newest and poorest residents nearest the center in a zone of transition and wealth rising steadily with distance from downtown. Hoyt's sector model (1939) says land use instead sorts into wedges radiating outward along transportation corridors, so that a city's wealthiest area is best described by direction from downtown rather than distance from it. Harris and Ullman's multiple nuclei model (1945) says a city of any real size develops several separate specialized centers rather than growing around one dominant core at all, each nucleus shaped by its own combination of shared-location benefits and land-use incompatibilities.

All three share the same underlying assumption worth stating plainly: land use is organized by a private, competitive land market in which the highest bidder for a given parcel determines what gets built there, and each model tries to predict the spatial pattern that competition produces. They diverge only in what they treat as the deciding geographic variable — pure distance from center for Burgess, direction along a transportation corridor for Hoyt, and functional clustering/repulsion around multiple independent centers for Harris and Ullman.

Why none of the three fits every city equally well

All three models were built from observing large American industrial cities in roughly the same fifty-year window, and that shared origin is exactly why each one struggles outside those conditions. They assume car-optional or car-dependent transportation systems typical of the mid-20th-century United States, a relatively unified private land market without strong state control over where development occurs, and an industrial-era economy organizing land use around factories, rail yards, and a single dominant downtown. Cities that developed under substantially different historical conditions — a different colonial history, a different transportation technology at the time of their main growth period, or a government that plans land use directly rather than leaving it mostly to private bidding — often do not match any of the three models cleanly.

This is exactly why geographers developed additional, region-specific models rather than treating the original three as universal. The Griffin-Ford model, describing many Latin American cities, keeps a strong central business district — often built around a colonial-era central plaza — but adds a distinctive commercial spine extending outward from it, lined by the city's most affluent housing, while quality of housing generally declines with distance from that spine and from the center, with the poorest residents occupying informal peripheral settlements at the urban edge. The McGee model, describing many Southeast Asian cities, replaces the single dominant CBD with a colonial-era port district as the historic anchor point, and organizes commercial activity into several distinct zones rather than one center, reflecting a different colonial economic history and a less centralized commercial core than the original three American-based models assume. A comparable regional model for Sub-Saharan African cities likewise typically describes multiple competing centers — often a colonial-era CBD, a traditional or indigenous market center, and a separate informal-commerce center — rather than the single dominant downtown Burgess's original model assumes.

The point of learning these regional variants is not to memorize a fourth, fifth, and sixth diagram for their own sake — it is to internalize the actual lesson: a spatial model is only as good as the assumptions baked into it, and applying the Burgess or Hoyt model to a city whose growth history looked nothing like early-20th-century Chicago will predict a pattern that city never actually developed.

Applying the models: worked comparisons

Chicago itself, the city Burgess actually studied, remains the model's best-fitting example precisely because its own data generated the model — a dense, historically dominant downtown Loop, with land use historically thinning into progressively lower-density, higher-income residential rings moving outward along the lakefront and beyond, matching the concentric logic reasonably well for the period Burgess observed it. A city built substantially around long-persisting directional wealth corridors — a lakefront, hillside, or park-facing high-income sector that has held its position across generations of new construction rather than relocating — illustrates Hoyt's sector logic better than a ring-based description would, since the defining variable there is which direction from downtown, not how far. A large, sprawling, polycentric metropolitan area with several genuinely separate major employment and commercial centers — an original downtown alongside one or more large secondary business districts miles away, each with its own office towers, retail, and surrounding development, none clearly subordinate to a single center — matches the multiple nuclei logic far better than trying to force it into one set of rings or one dominant wedge.

When an essay prompt asks you to evaluate which model best fits a described city, the strongest answers do not just name a model — they cite the specific evidence in the prompt that matches that model's defining mechanism, and explicitly note what the other two models would have predicted instead and why the evidence contradicts them. A useful thesis pattern for this kind of comparison: "While the concentric zone model predicts [ring-based prediction], [named city]'s [specific described feature] instead follows the sector model's logic, because [the specific mechanism — a persistent high-rent corridor along a particular route] better explains [the specific evidence given]." That structure does two things at once: it shows you understand more than one model well enough to rule the others out, and it ties your model choice to concrete evidence rather than a bare assertion.

Connecting the models back to the rest of the unit

The three internal-structure models are not standalone facts — they connect directly to nearly everything else in this unit. Burgess's zone of transition is where overurbanization's informal settlements and gentrification's disinvested, later-rediscovered neighborhoods both tend to concentrate. Hoyt's persistent high-rent sectors are a spatial explanation for why exclusionary zoning and housing-affordability pressure often cluster along the same corridors generation after generation. Harris and Ullman's specialized nuclei map closely onto the advanced-producer-service clusters and corporate-headquarters districts that make a city function as a node in the world-city network described earlier in this unit. Treat this unit, heading into the free-response section, as one connected argument about how competition for urban land produces observable, comparable, and explainable spatial patterns — not six separate vocabulary lists to memorize in isolation.

Why this matters for the exam

Free-response prompts on this unit very often hand you a data table, a map, or a written description of a real or hypothetical city and ask you to identify which model best explains it, with reasoning. The reasoning is worth more points than the naming: cite the model's defining mechanism (ring-based outward growth, corridor-based sector persistence, or multi-center specialization) and connect it explicitly to the evidence given, rather than simply asserting a model name. And do not forget the regional variants exist specifically to handle cities that don't fit the original three — if a prompt describes a colonial plaza with a commercial spine, or a port-anchored city with no single dominant downtown, recognizing that as Griffin-Ford or McGee territory rather than forcing it into Burgess or Hoyt is exactly the kind of higher-order distinction this exam rewards.

Practice: Free-Response Questions

Real AP-format prompts for this unit, each with a full model answer and the exact points a College Board reader would award. Click a question to see the answer — not AI-graded, just scored the way the real exam is scored.

LEQ

Respond to the following, using complete sentences.

Respond to the following, using complete sentences.

Urban geographers rely on several classic models — and related spatial theories — to describe how cities grow and how activities are arranged within them.

(a) Define urbanization.

(b) Describe the internal spatial arrangement of the concentric zone model, including where the central business district and the lowest-income housing are located relative to each other.

(c) Describe ONE specific way the sector model's internal spatial arrangement differs from the concentric zone model's.

(d) Describe the key organizing principle of the multiple-nuclei model — that is, explain why a city following this model develops several separate centers of activity rather than one.

(e) Explain the concepts of threshold and range as they are used in central place theory to determine which goods and services a settlement can support.

(f) Explain the difference between a country whose city-size distribution follows the rank-size rule and a country whose largest city is a primate city.

(g) A planner is studying a mid-sized city and observes that its wealthiest neighborhoods form a wedge extending outward from downtown along a scenic river corridor, while industrial land use forms a separate wedge along a rail line on the opposite side of downtown. Identify which urban land-use model (concentric zone, sector, or multiple-nuclei) best explains this city's layout, and explain your reasoning using the evidence given.

Model answer

(a) Urbanization is the process by which an increasing proportion of a population comes to live in cities and towns rather than in rural areas, accompanied by the physical growth and increasing economic/social importance of urban areas.

(b) In the concentric zone model, land uses are arranged as a series of rings that grow outward from a central business district (CBD) at the core. The lowest-income housing (along with the oldest and most deteriorated housing stock) sits in the zone immediately surrounding the CBD — the "zone in transition" — while household income generally increases as distance from the CBD increases, with the highest-income housing located in the outermost ring.

(c) The sector model differs by arranging land uses as wedges (sectors) radiating outward from the CBD along transportation routes, rather than as concentric rings. This means a single land-use type — such as a high-income residential district — extends the full distance from the center to the city's edge along one axis, instead of being confined to an outer ring, because high-value land uses grow outward along the same corridor over time once they are established near a route.

(d) The multiple-nuclei model holds that a city develops around several separate nodes (nuclei) of activity — such as a port, a university, an industrial park, and an outlying retail center — rather than a single dominant CBD. This happens because different activities have different locational needs and some activities benefit from clustering together (agglomeration) while others are incompatible and repel each other, so as a city grows, distinct specialized centers emerge independently around the features that attract them.

(e) Threshold is the minimum number of people (or minimum market size) needed to support a particular good or service being offered profitably at a location. Range is the maximum distance people are willing to travel to obtain that good or service. Together, threshold and range determine which goods a settlement of a given size can support: a good with a low threshold and short range (e.g., a convenience store) can be supported by many small settlements, while a good with a high threshold and long range (e.g., a specialty hospital) can only be supported by a large settlement that draws customers from a wide surrounding area.

(f) A country follows the rank-size rule when its cities' populations form a predictable, roughly even hierarchy: the nth-largest city has approximately 1/n the population of the largest city. This pattern typically reflects a well-developed, diversified national economy with functions distributed across many regional centers. A primate city, by contrast, is a country's largest city that is disproportionately larger than all other cities in the country (often more than twice the size of the second-largest) and dominates the country's economic, political, and cultural life. A primate-city distribution is common in countries with a colonial history or a highly centralized economy, where investment, infrastructure, and opportunity were concentrated in a single dominant city rather than spread among regional centers.

(g) This city's layout best fits the sector model. In the sector model, distinct land uses extend outward from the CBD as wedges along transportation corridors rather than as concentric rings, and the scenario describes exactly that pattern: high-income housing forms a wedge following a linear amenity corridor (the river) outward from downtown, while industrial land use forms a separate wedge following a different linear corridor (the rail line) on the opposite side. This is inconsistent with the concentric zone model, which would predict income increasing uniformly with distance from the CBD in all directions rather than concentrating along one specific corridor, and it does not describe multiple, functionally independent nuclei, so the sector model is the best fit.

Scoring · 7 points
1. Defines urbanization as the growing share of population living in/growth of cities.
2. Describes concentric zone model: rings from CBD, lowest income near center, income rises outward.
3. Describes at least one specific difference of the sector model (wedges along transportation routes vs. rings).
4. Describes the multiple-nuclei model's organizing principle: multiple independent specialized centers due to differing locational needs/agglomeration.
5. Explains both threshold and range in central place theory.
6. Explains the difference between rank-size rule distribution and a primate city.
7. Identifies the sector model for the described city and explains the reasoning using the wedge/corridor evidence given.
LEQ

Respond to the following, using complete sentences.

Respond to the following, using complete sentences. Use the data below to answer parts

(b) through (e).

Source: Population of the four largest cities in Country Y (hypothetical), most recent census.

| Rank | City | Population |
|---|---|---|
| 1 | Rivera | 8,400,000 |
| 2 | San Marcos | 950,000 |
| 3 | Belmar | 700,000 |
| 4 | Costilla | 610,000 |

(a) Define the rank-size rule.

(b) Using the rank-size rule, calculate the population that City rank 2 (San Marcos) would be expected to have if Country Y's city-size distribution followed the rank-size rule exactly.

(c) Compare the expected population calculated in part

(b) to San Marcos's actual population in the table, and state whether Country Y's city-size distribution follows the rank-size rule or instead displays a primate city. Justify your answer using the data.

(d) Explain what the pattern you identified in part

(c) suggests about the level of economic development and the distribution of political/economic power within Country Y.

(e) Rivera's downtown is described by planners as having high-rise, high-income condominiums immediately surrounding the CBD, with lower-income housing pushed increasingly farther from the center as distance from downtown increases. Identify which urban land-use model this pattern contradicts, and explain why.

(f) Explain ONE specific sustainability challenge that a rapidly growing primate city like Rivera is likely to face as a result of its disproportionate population concentration.

(g) Propose ONE specific urban planning policy that Rivera's government could adopt to address the challenge you identified in part (f), and explain how that policy would help.

Model answer

(a) The rank-size rule is a pattern observed in some countries' urban hierarchies in which the population of a city is inversely proportional to its rank among all cities in the country — the population of the nth-ranked city is approximately equal to the population of the largest city divided by n.

(b) Under the rank-size rule, the rank-2 city's expected population = (population of rank-1 city) / 2 = 8,400,000 / 2 = 4,200,000.

(c) San Marcos's actual population (950,000) is far below the 4,200,000 that the rank-size rule would predict — it is only roughly one-ninth of Rivera's population rather than one-half. Because Rivera is dramatically larger than would be expected relative to the rest of the country's cities, Country Y displays a primate-city pattern rather than a rank-size distribution: Rivera dominates the urban hierarchy instead of the city sizes stepping down in the smooth, predictable way the rank-size rule describes.

(d) A primate-city pattern like this typically indicates that Country Y's economic development is uneven and centralized rather than diversified — investment, industry, government administration, transportation infrastructure, and job opportunities are concentrated heavily in Rivera rather than spread among regional centers. This pattern is common in countries with a colonial history (where the colonial power built up a single administrative/export hub) or in countries where post-independence government and economic policy continued to funnel resources into one dominant city, leaving secondary cities comparatively underdeveloped and lacking the infrastructure or economic base to grow into true regional counterweights.

(e) This pattern contradicts the concentric zone model. In the concentric zone model, the zone immediately surrounding the CBD is the "zone in transition," associated with the oldest housing stock and the lowest-income residents, with income rising as distance from the CBD increases outward toward the city's edge. Rivera's description is the reverse of that — high-income housing is closest to the CBD and income falls with distance — so it does not fit the concentric zone model's predicted income gradient.

(f) One likely sustainability challenge is severe strain on housing, infrastructure, and services: because so much of the national population and economic activity is concentrated in Rivera, the city is likely to face overcrowding, informal/unplanned settlement growth on its periphery, traffic congestion, inadequate sanitation and water systems, and air pollution, as infrastructure investment struggles to keep pace with population growth concentrated in one place rather than being distributed across multiple smaller, more manageable urban centers.

(g) Rivera's government could adopt a policy of decentralization — deliberately relocating government offices, universities, or incentivizing private industry to open new offices/factories in secondary cities such as San Marcos or Belmar. This would help by redirecting job opportunities and investment away from Rivera toward the smaller cities, reducing migration pressure into Rivera, easing the strain on its housing and infrastructure, and helping the country's urban hierarchy move toward the more balanced, rank-size pattern described in part (a).

Scoring · 7 points
1. Defines the rank-size rule.
2. Correctly calculates the expected rank-2 population as 4,200,000.
3. Compares expected vs. actual population and correctly identifies a primate-city pattern with data-based justification.
4. Explains what the primate-city pattern suggests about economic development/centralization of power.
5. Identifies the concentric zone model as contradicted and explains why (income gradient reversed).
6. Explains a specific sustainability challenge tied to primate-city concentration.
7. Proposes a specific, plausible planning policy and explains how it addresses the challenge.
LEQ

Respond to the following, using complete sentences.

Respond to the following, using complete sentences. Use both sources below to answer the parts that follow.

Source 1: Planning report excerpt, City of Harlow (hypothetical). "Harlow's manufacturing and warehousing land use forms a continuous wedge running outward from downtown along the freight rail line on the city's east side. A separate wedge of upper-income housing extends outward from downtown along the west-side ridge, where homes have long enjoyed unobstructed views and easy highway access. Middle-income housing fills the areas between these two wedges."

Source 2: Population and built-up land area of Harlow's metropolitan region, two censuses twenty years apart.

| Year | Metro population | Built-up land area (sq. mi.) |
|---|---|---|
| Year 1 | 480,000 | 140 |
| Year 20 | 690,000 | 310 |

(a) Using Source 1, identify which urban land-use model best describes Harlow's internal spatial structure.

(b) Explain your answer to part (a), citing specific evidence from Source 1.

(c) Identify ONE aspect of Harlow's land use that the model you identified in part

(a) does NOT fully explain, based on what is described in Source 1.

(d) Using Source 2, describe the relationship between Harlow's population growth and its growth in built-up land area over the twenty-year period.

(e) Explain the term used for the pattern you described in part (d), and explain ONE specific consequence this pattern typically has for a metropolitan region's transportation systems or farmland/open space at its periphery.

(f) Explain how the wedge-shaped growth pattern described in Source 1 likely contributed to the land-use trend shown in Source 2.

(g) Propose ONE specific urban planning strategy Harlow could adopt to address the trend shown in Source 2, and explain how it would help slow the disproportionate growth in built-up land relative to population.

Model answer

(a) Harlow's internal spatial structure best fits the sector model.

(b) Source 1 describes land uses forming wedges that extend outward from downtown along transportation and amenity corridors — manufacturing along the rail line and upper-income housing along the ridge/highway corridor — rather than as concentric rings around the CBD. This wedge-along-a-corridor pattern, with middle-income housing filling the space between the two sectors, is the defining characteristic of the sector model, in which land uses grow outward from the center along a fixed axis once established near a transportation route.

(c) The sector model, as classically described, does not fully explain why manufacturing/warehousing (an industrial use) and upper-income housing (a residential use) each cluster into their OWN separate, specialized wedge rather than simply being separated by income alone — the presence of two functionally distinct specialized wedges (one industrial, one residential) organized around different infrastructure (rail vs. highway/ridge) is better explained by the multiple-nuclei model's idea that different activities cluster around different specialized nodes/corridors based on their own locational needs.

(d) Over the twenty-year period, Harlow's metro population grew by about 44% (from 480,000 to 690,000), while its built-up land area grew by about 121% (from 140 to 310 square miles) — the land area consumed by development grew more than twice as fast, proportionally, as the population did.

(e) This pattern is called urban sprawl — low-density outward expansion of a metropolitan area's built-up footprint at a rate that outpaces its population growth. One specific consequence of sprawl is the loss of farmland and open space at the metropolitan periphery, as low-density subdivisions, roads, and commercial strips consume land that was previously used for agriculture; another common consequence is increased automobile dependence and traffic congestion, since low-density, spread-out development is difficult to serve efficiently with public transit and forces most trips to be made by car.

(f) The wedge-shaped growth pattern in Source 1 is organized around linear transportation corridors (the rail line, the highway), and because sector-model growth extends outward along a corridor rather than filling in a compact ring around downtown, new development can continue stretching farther and farther from the CBD along each corridor. This corridor-driven, linear growth is inherently land-extensive rather than land-efficient, which helps explain why Harlow's built-up area expanded so much faster than its population — growth was not being absorbed through denser infill closer to downtown but was instead consuming new land at the leading edge of each wedge.

(g) Harlow could adopt a smart-growth or urban growth boundary policy — for example, establishing a boundary that limits new large-lot development beyond a certain distance from downtown while incentivizing higher-density, mixed-use infill development closer to the existing built-up area and along transit corridors. This would help by directing new population growth into already-developed areas rather than allowing it to consume additional peripheral land, which would slow the rate of built-up land expansion relative to population growth and help preserve remaining farmland and open space at the metro edge.

Scoring · 7 points
1. Identifies the sector model from Source 1.
2. Explains the identification using specific wedge/corridor evidence from Source 1.
3. Identifies a specific limitation of the sector model (e.g., doesn't explain the separate specialized industrial vs. residential wedges) tied to Source 1.
4. Describes the population vs. built-up land area growth relationship from Source 2 (land area grew much faster than population).
5. Names the pattern as urban sprawl and explains one specific consequence (farmland/open-space loss OR transportation/auto-dependence).
6. Explains how the Source 1 wedge/corridor growth pattern contributed to the Source 2 sprawl trend.
7. Proposes a specific planning strategy addressing the trend and explains how it would help.
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