Why Cities Grow
Start with why cities form, expand, and concentrate people.
Exam focus
What You Must Know for Unit 6
AP Human Geography Unit 6 rewards students who connect urban processes to visible spatial patterns. Before test day, you should be able to explain the following:
- Why cities grow — start with urbanization and site and situation of cities
- How settlements form urban systems — urban hierarchy, Central Place Theory, and rank-size rule vs primate city
- How city models explain land use — see urban land-use models
- Why suburbs, sprawl, and edge cities develop — suburbanization and urban sprawl
- How gentrification and sustainability shape modern cities — gentrification and sustainable cities
For spatial tools, review Unit 1 thinking geographically. Migration into cities links to Unit 2 population and migration. Neighborhood culture ties to Unit 3 cultural patterns. Capitals and governance connect to Unit 4 political patterns. Rural-to-urban land competition compares with Unit 5 agriculture and rural land use. More practice: daily practice, practice by course, practice by topic, and practice tests.
Speed review
The 60-Second Unit 6 Overview
Think of Unit 6 as three questions: Why do cities exist? How are cities arranged? What problems do cities create or solve?
Your path through Unit 6
Your Unit 6 Learning Journey
Follow this order so city systems, urban models, and urban change build logically.
How Cities Connect
Learn how cities fit into larger settlement systems.
How Cities Are Organized
Master the models that explain land use inside cities.
How Cities Spread and Change
Explain outward growth, suburbs, neighborhood change, and displacement.
How Cities Solve Problems
Connect urban growth to planning, sustainability, and policy.
Work through all five groups, then use practice MCQs and FRQ strategy on this hub to lock in exam skills.
Unit 6 Concept Map
Urbanization → Urban Hierarchy → City Models → Suburbanization → Urban Problems → Sustainable Solutions
Urbanization
People and jobs concentrate in cities as rural-to-urban migration and natural increase reshape settlement.
Exam clue: Name economic pull factors, not only bigger totals.
Urban Hierarchy
Hamlets to world cities rank by population and service level.
Exam clue: Higher-order functions need larger market areas.
City Models
Models explain internal land-use patterns from the CBD outward.
Exam clue: Match shape on the map to the right model name.
Suburbanization
Low-density growth spreads along highways beyond the core.
Exam clue: Separated zoning plus cars stretch the city outward.
Urban Problems
Sprawl, segregation, decay, and informal settlements create uneven burdens.
Exam clue: Pair problem with mechanism, not moral judgment alone.
Sustainable Solutions
Transit, density, mixed use, and growth boundaries reduce harm.
Exam clue: Smart growth targets cause and spatial outcome.
Core Concepts
Fifteen high-yield ideas with definitions, exam clues, and common mistakes.
Urbanization
Growth of cities as people and jobs concentrate in urban areas.
AP exam clue: Name push (rural poverty) and pull (jobs, services) factors—not only population totals.
Common mistake: Calling all migration urbanization without rural-to-urban movement.
Site and Situation
Site is local physical traits; situation is connections to other places.
AP exam clue: Harbor depth = site; highway junction = situation.
Common mistake: Using site and situation as synonyms.
Urban Hierarchy
Settlements ranked by population and service functions.
AP exam clue: Higher-order services cluster in larger centers.
Common mistake: Assuming biggest city has every function without evidence.
Central Place Theory
Christaller model of settlement spacing for goods and services.
AP exam clue: Threshold and range predict where functions locate.
Common mistake: Confusing range with threshold.
Rank-Size Rule
City n is often about 1/n the size of the largest city.
AP exam clue: Fourth city ≈ one-quarter of the largest in balanced systems.
Common mistake: Applying rank-size when one city dominates.
Primate City
One metro far larger than the second—often 2×+.
AP exam clue: Paris or Buenos Aires style dominance on national maps.
Common mistake: Calling any capital a primate city without size data.
World Cities
Global command centers for finance, law, and corporate HQ.
AP exam clue: London–New York–Tokyo style producer services.
Common mistake: Equating world city with megacity by population only.
Megacities and Metacities
Megacity ≈ 10M+; metacity = fused metro complexes.
AP exam clue: Lagos or Dhaka scale vs. polycentric Pearl River Delta.
Common mistake: Megacity label does not guarantee global-city functions.
CBD
Central business district—peak accessibility and bid rent.
AP exam clue: Skyscrapers and flagship retail signal CBD land competition.
Common mistake: Treating CBD as the entire municipality.
Concentric Zone Model
Burgess rings radiate from the CBD.
AP exam clue: Rings around core—classic textbook diagram.
Common mistake: Expecting perfect circles in every real city.
Sector Model
Hoyt wedges along transport corridors.
AP exam clue: Industry and housing stretch along rails or highways.
Common mistake: Mixing sector wedges with concentric rings.
Multiple Nuclei Model
Several specialized nodes instead of one core.
AP exam clue: Airport, university, or mall as mini-centers.
Common mistake: Forcing a single CBD when nodes are listed.
Galactic City Model
Suburban edge cities on beltways around a core.
AP exam clue: Office parks and malls on orbital highways.
Common mistake: Calling all suburbs galactic cities.
Suburbanization and Urban Sprawl
Low-density outward growth, often car-dependent.
AP exam clue: Leapfrog lots and separated land uses.
Common mistake: Equating all suburban growth with sprawl.
Gentrification
Reinvestment raising values; displacement risk for renters.
AP exam clue: New cafes plus rising rents in one paragraph.
Common mistake: Praising renewal without displacement harms.
Urban Models Comparison
Compare the four core U.S.-style models, then open each full guide for map clues and FRQ language.
| Model | Main Idea | Shape/Pattern | Best Clue | Common AP Mistake |
|---|---|---|---|---|
| Concentric Zone | Rings of land use by distance from CBD | Circular zones | Rings around core | Expecting perfect circles everywhere |
| Sector | Wedges along transport routes | Sector wedges | Corridors from CBD | Confusing with concentric rings |
| Multiple Nuclei | Several specialized centers | Scattered nodes | Airport, university, mall nodes | Forcing one monocentric CBD |
| Galactic / Peripheral | Edge cities on beltways | Suburban employment poles | Office parks on orbital highways | Calling all suburbs edge cities |
| Latin American | Elite spine and informal periphery | Spine + sector mix | Mansion zone plus squatter belts | Ignoring colonial core layering |
| Southeast Asian | Port-led export zones | Port + dense corridors | Factories along transit to port | Treating like U.S. sprawl only |
| African | Dual CBD and informal growth | Split cores + periphery | Colonial admin vs. market district | Single-center assumption |
Open the urban land-use models study guide for side-by-side diagrams and practice.
Model Identification Drill
Read each clue, reveal the answer, then open the full topic guide.
Rings around the CBD
Answer: Concentric Zone Model
Why: Burgess’s model organizes land use in rings expanding outward from the central business district.
Wedges along transport routes
Answer: Sector Model
Why: Hoyt’s model shows housing and industry stretching in corridors along rails, roads, or rivers.
Several activity nodes
Answer: Multiple Nuclei Model
Why: Harris and Ullman argued cities develop multiple specialized centers instead of one core.
Highways, suburbs, and edge cities
Answer: Galactic City Model
Why: Decentralized employment and retail on beltways fit automobile-oriented metropolitan sprawl.
Global finance and command functions
Answer: World City
Why: World cities anchor multinational headquarters and advanced producer services—not size alone.
Central Place Theory Made Simple
What is Central Place Theory?
Central Place Theory explains how settlements space themselves to provide goods and services to surrounding market areas. Higher-order functions need larger thresholds and wider ranges than everyday low-order goods like groceries.
- Central place — settlement providing goods and services
- Hinterland — surrounding market area served
- Threshold — minimum demand to support a function
- Range — maximum distance consumers will travel
- High-order goods — specialized, infrequent (cardiac surgery)
- Low-order goods — frequent, everyday (milk, coffee)
- Hexagons — efficient nesting of market areas on flat plains
Study Central Place Theory in depth
Choose the higher-order service
Heart surgery hospital because it needs a larger threshold and range.
Rank-Size Rule vs Primate City
What is the difference between rank-size rule and primate city?
Rank-size describes a balanced urban system where the nth city is often about 1/n the size of the largest. A primate city pattern exists when one metro dominates—often more than twice the second city—in economy and culture.
Rank-size rule: In a balanced system, the nth city is often about 1/n the population of the largest—fourth city ≈ one-quarter of the largest.
Primate city: One metro dominates national economy and culture—often more than twice the second city (Paris, Buenos Aires, Bangkok examples on exams).
Simple example: Country A has cities of 8M, 4M, 2M, 1M (rank-size-like). Country B has 12M, 2M, 1.5M, 1M (primate).
AP exam clue: Population tables—compute ratios before you label primate vs rank-size.
Do not confuse: Rank-size describes a pattern across many cities; primate describes dominance by one city.
World Cities vs Megacities
What is the difference between a world city and a megacity?
A megacity is defined chiefly by very large population—often ten million or more. A world city is defined by command-and-control functions such as global finance, corporate headquarters, and advanced producer services.
World cities concentrate global finance, law, media, and corporate headquarters. Megacities and metacities are defined chiefly by population scale—often ten million people or more—with metacities describing fused metropolitan regions.
AP exam clue: Command functions signal world cities; headcount alone signals megacity.
Sprawl, Suburbanization, and Gentrification
What is urban sprawl?
Urban sprawl is low-density outward growth from a city, often shaped by highways, car dependence, and separated single-use zoning. It can lengthen commutes, raise infrastructure costs, and increase vehicle miles traveled.
Suburbanization and urban sprawl spread low-density housing and separated land uses along highways, raising commute times and infrastructure costs.
What is gentrification in AP Human Geography?
Gentrification is reinvestment and demographic change in a lower-income neighborhood as higher-income residents and businesses arrive, often raising property values and rents. Strong answers note improved services alongside displacement risk.
Gentrification brings reinvestment, new businesses, and rising property values while increasing displacement risk for long-term renters.
Urban Problems and Solutions
What are sustainable cities?
Sustainable cities use planning tools such as transit-oriented development, mixed use, density, green space, and growth boundaries to reduce pollution, congestion, and unequal environmental burdens while meeting housing needs.
Problem: Sprawl
Solution: Smart growth / urban growth boundaries
Solution: Smart growth / urban growth boundaries
Problem: Displacement
Solution: Affordable housing policy
Solution: Affordable housing policy
Problem: Traffic congestion
Solution: Transit-oriented development
Solution: Transit-oriented development
Problem: Segregation
Solution: Fair housing / environmental justice
Solution: Fair housing / environmental justice
Problem: Inner-city decay
Solution: Revitalization with anti-displacement protections
Solution: Revitalization with anti-displacement protections
Problem: Informal settlements
Solution: Infrastructure and legal recognition
Solution: Infrastructure and legal recognition
How to Write a Unit 6 FRQ
Formula: Cause → spatial pattern → consequence (identify the urban process, explain the map pattern, connect to a real outcome).
Example prompts
- Gentrification FRQ — reinvestment, rising rents, displacement risk, changed services
- Sprawl FRQ — highways and zoning, VMT, infrastructure cost, environmental footprint
- Central Place Theory FRQ — threshold, range, hierarchy
- City model FRQ — map clue → model name → why transport or history shaped it
Sentence starters
- “One spatial pattern shown is…”
- “This occurs because…”
- “A likely consequence is…”
- “This model fits because…”
Common Unit 6 Mistakes
World city ≠ megacity
Wrong idea: Any huge city is automatically a world city.
Better idea: World cities command global services; megacities are size-defined.
AP clue: Look for finance/HQ language, not population alone.
Rank-size rule ≠ primate city
Wrong idea: Using the terms interchangeably.
Better idea: Rank-size is a regular ladder; primate is one dominant rung.
AP clue: Compare largest to second city before you label.
Suburbanization ≠ urbanization
Wrong idea: All growth outside the core is urbanization.
Better idea: Urbanization is shift to urban areas; suburbanization is outward movement within metros.
AP clue: Name direction and density, not only “more people.”
Gentrification ≠ urban revitalization only
Wrong idea: Revitalization always helps every resident.
Better idea: Gentrification includes displacement risk; revitalization is the broader renewal label.
AP clue: Balance benefits and harms in one answer.
Sector model ≠ concentric zone model
Wrong idea: Both are only rings.
Better idea: Sectors follow corridors; concentric follows distance from CBD.
AP clue: Transport lines are the sector giveaway.
Range ≠ threshold
Wrong idea: Swapping the two definitions.
Better idea: Threshold = minimum customers; range = max travel distance.
AP clue: Daily milk vs. rare surgery hospital.
CBD ≠ entire city
Wrong idea: Calling the whole municipality the CBD.
Better idea: CBD is the peak-accessibility core, not suburbs or edge cities.
AP clue: Skyscrapers and bid rent signal CBD.
Sprawl ≠ all suburban growth
Wrong idea: Any suburb equals sprawl.
Better idea: Sprawl is low-density, leapfrog, separated-use outward growth.
AP clue: Highway dependence and infrastructure cost.
Unit 6 Vocabulary Preview
Showing all 35 terms
urbanization
Shift of population and economic activity toward cities.
urban hierarchy
Ranking settlements by size and service level.
central place theory
Model of how settlements space services across a landscape.
threshold
Minimum demand needed to support a function.
range
Maximum distance consumers travel for a good or service.
hinterland
Surrounding area served by a central place.
rank-size rule
Regular size steps down the urban system.
primate city
Dominant city far larger than the second.
world city
Global hub for advanced producer services.
megacity
Metro area over roughly ten million people.
metacity
Merged metropolitan complex exceeding megacity scale.
CBD
Central business district with peak land values.
urban morphology
Physical form and layout of a city.
concentric zone model
Ring pattern expanding from the CBD.
sector model
Wedge-shaped zones along transport routes.
multiple nuclei model
Several specialized activity nodes.
galactic city model
Edge cities on suburban beltways.
suburb
Residential area beyond the historic core.
edge city
Suburban employment and retail concentration.
boomburb
Fast-growing large suburb.
exurb
Low-density fringe tied to a metro by long commutes.
urban sprawl
Low-density, separated, outward expansion.
gentrification
Reinvestment changing neighborhood class and rents.
blockbusting
Speculative panic selling for racial turnover.
white flight
Departure of white residents to suburbs.
redlining
Historic lending maps starving minority areas of credit.
segregation
Spatial sorting by race or income.
inner-city decay
Deterioration of aging core neighborhoods.
urban revitalization
Public or private renewal of urban districts.
squatter settlement
Unauthorized housing on contested land.
informal settlement
Housing built outside formal regulation.
New Urbanism
Walkable, mixed-use neighborhood design.
smart growth
Compact, transit-friendly development policy.
urban growth boundary
Legal limit on outward urban expansion.
environmental justice
Fair distribution of environmental burdens and benefits.
Unit 6 Practice Preview
Five sample MCQs with shuffled choices and explanations. Use FRQ strategy for free-response planning.
Central Place Theory
Question 1
Central place theory predicts fewer cardiac surgery centers than corner stores mainly because hospitals need:
Correct: B. Higher-order services need more patients drawn from farther away—larger threshold and range.
Why not A: Smaller thresholds fit low-order daily goods like milk, not rare surgery.
AP clue: Match service type to threshold and range before you pick a settlement size.
Rank-size rule
Question 2
In a balanced urban system, the fourth-largest city is often about what fraction of the largest?
Correct: C. Rank-size expects population of rank n ≈ largest ÷ n.
Why not A: 1/16 would imply an extremely steep drop not typical of rank-size balance.
AP clue: Compute largest ÷ rank before you label primate versus rank-size.
Primate city
Question 3
Which scenario best illustrates a primate city?
Correct: A. Primacy shows overwhelming dominance of one metro over the rest.
Why not B: Similar regional capitals suggest a balanced ladder, not primacy.
AP clue: Compare the largest city to the second city before you name the pattern.
World city vs megacity
Question 4
A world city is best defined by:
Correct: B. World/global cities anchor multinational HQ and advanced services—not size alone.
Why not A: Ten million people defines a megacity, not global command functions.
AP clue: Look for finance, headquarters, and producer services—not population alone.
City models
Question 5
Ring-shaped zones expanding from a CBD describe which model?
Correct: B. Burgess concentric zones radiate outward from the core.
Why not A: Sectors are wedges along transport routes, not full rings.
AP clue: Rings around the CBD signal Burgess; wedges signal Hoyt.
Mini FRQ practice
Explain how highway expansion and separated-use zoning can contribute to urban sprawl in one metropolitan area. Use cause → pattern → consequence.
Compare your outline to the Unit 6 FRQ strategy section before you move on.
More AP HUG practice: daily practice, by course, by topic, and practice tests.
Mini Diagnostic: Can you do these before test day?
Unit 6 FAQ
What is AP Human Geography Unit 6 about?
AP Human Geography Unit 6 studies how cities form, grow, organize land, connect to other places, and respond to urban problems. The unit focuses on urbanization, settlement hierarchy, city models, central place theory, rank-size rule, primate cities, suburbanization, sprawl, gentrification, and sustainable urban planning.
What are the most important Unit 6 models?
Expect Burgess concentric zones, Hoyt sectors, Harris–Ullman multiple nuclei, and galactic or peripheral sprawl patterns. Match each model to transport history and where housing, industry, and services concentrate.
What is Central Place Theory?
Christaller-style central place theory explains how settlements space themselves to serve surrounding areas with goods and services. Threshold is minimum demand to keep a function profitable; range is how far people travel; higher-order functions need larger thresholds and wider ranges.
What is the difference between rank-size rule and primate city?
Rank-size describes a fairly regular urban system where the nth city is often about 1/n the size of the largest. A primate pattern exists when one metro dominates—often more than twice the second city—because power and services concentrate there.
What is the difference between a world city and a megacity?
A megacity is defined chiefly by very large population—often ten million or more. A world city is defined by command-and-control functions: finance, corporate headquarters, and advanced producer services.
What is urban sprawl?
Urban sprawl is low-density outward growth from a city, often shaped by highways, car dependence, single-use zoning, and suburban housing. It can increase commute times, infrastructure costs, farmland loss, and vehicle miles traveled.
What is gentrification in AP Human Geography?
Gentrification is reinvestment and demographic change in a lower-income neighborhood as higher-income residents and businesses arrive, often raising property values and rents. Balanced AP responses note improved services alongside displacement risk.
What are sustainable cities?
Sustainable cities use planning tools such as transit-oriented development, mixed use, density, green space, and growth boundaries to reduce pollution, congestion, and unequal environmental burdens while still meeting housing and economic needs.
How do you study for AP Human Geography Unit 6?
Follow the Unit 6 learning journey on this hub, practice matching map clues to city models, drill threshold versus range, compare rank-size and primate city with real countries, and write short FRQs using cause → pattern → consequence.
What are common Unit 6 FRQ topics?
Common prompts apply threshold and range, justify a city model from a description, explain gentrification tradeoffs, analyze sprawl drivers and sustainability responses, or interpret urban hierarchy from population tables.