Harris and Ullman's multiple nuclei model (1945) is an urban structure model proposing that cities develop around several separate centers of activity, or nuclei, such as a CBD, industrial districts, universities, and airports, rather than growing outward from a single central business district.
The multiple nuclei model, created by geographers Chauncy Harris and Edward Ullman in 1945, argues that a city doesn't have just one center. Instead, it has several. The original CBD is still there, but other 'nuclei' grow up around their own activities. A port attracts warehouses. A university attracts bookstores and apartments. An airport attracts hotels. Heavy industry attracts working-class housing nearby (and pushes wealthy housing far away, because nobody wants to live next to a factory).
The big idea behind the model is that some land uses attract each other while others repel each other. Similar activities cluster to share benefits, and incompatible activities (like factories and high-income neighborhoods) keep their distance. That makes the multiple nuclei model the most flexible and realistic of the three classic models in the CED. Burgess drew rings, Hoyt drew wedges, but Harris and Ullman basically said cities are messier than that, especially once cars and trucks made it easy to do business away from the old downtown.
This model lives in Topic 6.5, The Internal Structure of Cities, in Unit 6. It directly supports learning objective AP Human Geography 6.5.A, which asks you to explain the internal structure of cities using various models and theories. The CED (EK PSO-6.D.1) names the Harris and Ullman multiple-nuclei model alongside the Burgess concentric-zone model, the Hoyt sector model, the galactic city model, bid-rent theory, and regional models from Latin America, Southeast Asia, and Africa. You're expected to know all of them and, more importantly, to compare them. The multiple nuclei model is the bridge in that story. It's the moment urban theory admitted that the single-CBD assumption was breaking down, which sets up the galactic city model and edge cities later in Unit 6.
Keep studying AP Human Geography Unit 6
Burgess Concentric Zone Model (Unit 6)
Burgess assumed one CBD with rings of land use expanding outward. Harris and Ullman directly challenged that assumption by saying a city can have many centers. On the exam, the fastest way to tell the models apart is to count the centers.
Sector Model (Unit 6)
Hoyt's sector model kept the single CBD but let land uses grow outward in wedges along transportation lines. The multiple nuclei model goes one step further and drops the single-center idea entirely. Think of the three classic models as a timeline of cities getting more car-dependent and more spread out.
Edge Cities (Unit 6)
Edge cities are the multiple nuclei model playing out in real life. When offices, malls, and hotels cluster near a suburban highway interchange, that's a new nucleus forming far from the old downtown. The galactic city model takes this idea and builds a whole model around it.
Central Business District (CBD) (Unit 6)
The multiple nuclei model doesn't delete the CBD. It demotes it. The downtown is still one nucleus, often the biggest, but it no longer monopolizes commerce the way it does in Burgess's rings.
Multiple-choice questions usually test this model in one of two ways. Either you get a diagram or description of a city and have to identify which model fits, or you get a scenario (a new shopping district forming around an airport, for example) and have to pick the model that explains it. The giveaway for multiple nuclei is always more than one center of activity, or land uses clustering and repelling each other. On the FRQ side, no released question has used the term verbatim, but Topic 6.5 models are classic free-response material. You should be ready to describe the model, explain why it fits modern car-based cities better than Burgess or Hoyt, and identify its limitations (it was built on mid-20th-century American cities, so it doesn't map neatly onto cities in Latin America, Southeast Asia, or Africa, which have their own CED models).
Both models feature multiple centers, so they blur together fast. The difference is era and emphasis. Multiple nuclei (1945) describes a city with several nuclei that grew up around different activities within the urban area. The galactic city model (also called the peripheral model) describes the post-1970s American metro, where edge cities orbit a declining original CBD and everything is connected by beltway highways. If the question stresses suburban nodes ringed around a city and linked by a highway loop, that's galactic. If it stresses separate activity centers attracting compatible land uses, that's multiple nuclei.
Harris and Ullman proposed the multiple nuclei model in 1945, arguing that cities grow around several centers of activity instead of a single CBD.
Each nucleus forms around a specific function, like a port, university, airport, or industrial park, and attracts compatible land uses while repelling incompatible ones.
The model reflects the rise of the automobile, which let businesses and people locate away from the old downtown core.
It is one of six models named in EK PSO-6.D.1 for explaining the internal structure of cities, alongside Burgess, Hoyt, the galactic city model, bid-rent theory, and regional models.
The quickest way to compare the classic models is by shape and center count. Burgess has rings around one center, Hoyt has wedges from one center, and Harris and Ullman have multiple centers.
The model's main limitation is that it was based on mid-20th-century American cities, so it does not describe cities in Latin America, Southeast Asia, or Africa as well as the region-specific models do.
It's a 1945 urban model stating that cities develop around multiple separate centers (nuclei) of activity, such as the CBD, industrial zones, universities, and airports, rather than expanding outward from one downtown. It appears in Topic 6.5 under EK PSO-6.D.1.
Yes. The CBD is still one of the nuclei, often the largest, but it's no longer the only center of economic activity. The model demotes the CBD rather than eliminating it.
Multiple nuclei (1945) describes several activity centers within a city, formed because some land uses attract each other and others repel. The galactic city model describes the later edge-city era, where suburban business nodes orbit a declining downtown along beltway highways. Galactic is essentially the multiple nuclei idea pushed out to the suburban fringe.
They didn't fully reject them, but by 1945 the automobile had made the single-CBD assumption unrealistic. Cars and trucks let factories, stores, and homes locate far from downtown, so cities developed multiple specialized centers instead of neat rings or wedges around one core.
Not very well, and the CED accounts for that. The model was built on mid-20th-century American cities, so EK PSO-6.D.1 also lists separate models for Latin American, Southeast Asian, and African cities. Knowing this limitation is a common way exams test the model.