Airline hubs are specific airports that an airline company uses as a transfer sport to get travelers to their intended destination. This system is part of the spoke-hub transportation model, which has been favored by many airlines since the industry was deregulated in 1978. The model consists of a system of connections in which all airline traffic moves across spokes linked to the hub at the center.For example, Denver and Los Angeles represent hubs, with many spokes reaching outward to other cities. The purpose of this model is simple: to save airlines money and to provide passengers better routes to their selected destinations. Today, most airlines have at least one main airport that their flights must travel through, and from there, the flights go outwards on different network spokes.So which airlines dominate which airports? Let’s take a look at seven major U.S. airlines, and see where their hubs are located.American Airlines AALThe world’s largest airline by fleet size and revenue, American Airlines has been an industry staple since its formation in 1930. The company is headquartered in Fort Worth, Texas, and since its merger with US Airways in 2013, American Airlines operates nine domestic hubs:Dallas-Fort Worth International AirportCharlotte-Douglas International AirportChicago O’Hare International AirportPhiladelphia International AirportPhoenix Sky Harbor International AirportMiami International AirportRonald Reagan Washington International AirportLos Angeles AirportNew York City’s John F. Kennedy International AirportAlaska Airlines ALKAlaska Airlines is American airline founded back in 1932. Originally offering flights from Anchorage, Alaska, the airlines today has flights to more than 100 destinations including Alaska, Hawaii, Canada, Costa Rica, and Mexico. Alaska Airlines’ fleet consists of mostly Boeing (BA) 737 aircraft, as well as Bombardier and Embraer planes. The company has three main hubs:Seattle-Tacoma International AirportPortland International AirportTed Stevens Anchorage International AirportAnd two focus cities:San Diego International AirportSan Jose International AirportDelta Airlines DALLike American, Delta has been present in the U.S. airline industry for decades. It began carrying passengers in 1929, and has grown rapidly since then due to many airline mergers. Headquartered in Atlanta, Georgia, Delta operates 10 domestic hubs:Hartsfield-Jackson Atlanta International AirportDetroit Metropolitan Wayne County AirportCincinnati/Northern Kentucky International AirportJFK International AirportNew York City’s La Guardia AirportBoston Logan International AirportLos Angeles International AirportMinneapolis-St. Paul International AirportSalt Lake City International AirportSeattle-Tacoma International AirportAnd three international hubs:Amsterdam Airport SchipholTokyo Narita International AirportParis-Charles de Gaulle AirportFrontier AirlinesFounded in 1994, Frontier Airlines is a popular, privately-held, ultra-low-cost carrier headquartered in Denver Colorado. It recently announced a huge route expansion, and Frontier is adding 21 new destinations and 85 new routes to its network. Additionally, Frontier is adding dozens of non-stop flights between other cities that it already serves. With this network increase, Frontier’s route map will grow to 82 cities by next summer. The airline operates out of one main hub:Denver International AirportAnd seven focus cities:Cincinnati/Northern Kentucky International AirportCleveland Hopkins International AirportHartsfield-Jackson Atlanta International AirportLas Vegas’ McCarran International AirportOrlando International AirportPhiladelphia International AirportTrenton-Mercer AirportJetBlue Airways JBLUJetBlue is a popular low-cost airline option headquartered in Long Island in New York City. It serves 97 destinations in the U.S., Mexico, the Caribbean, Central America, and South America. JetBlue was founded in 1998, and primarily uses Airbus and Embraer aircraft. JetBlue utilizes the old0fashioned point-to-point system, which carries passengers short distances with few connecting flights, the airline operates out of six domestic focus cities:JFK International AirportFort Lauderdale-Hollywood International AirportBoston Logan International AirportLong Beach AirportSan Juan’s Luis Munoz Marin International AirportOrlando International AirportSouthwest Airlines LUVTouted as the world’s largest low-cost carrier, Southwest Airlines was launched in 1967 and is known for its Rapid Rewards frequent-flyer program. Its headquarters is in Dallas, Texas. While Southwest does not use the hub and spoke transportation model (it prefers the point-to-point system like JetBlue) the airline still operates out of 10 major domestic cities:Chicago Midway International AirportBaltimore-Washington International AirportMcCarran International AirportDallas Love Field AirportDenver International AirportPhoenix Sky Harbor International AirportHouston’s William P. Hobby AirportHartsfield-Jackson Atlanta International AirportOrlando International AirportLos Angeles International AirportUnited Continental Holdings UALHeadquartered in Chicago, United Continental Holdings is an airline holding company for United Airlines and Continental Airlines, which formed in a merger back in 2010. Operating more than 700 mainline aircraft, United Continental needs a lot of hubs. The airline company has nine hubs in total, eight domestic:Chicago O’Hare International AirportHouston’s George Bush Intercontinental AirportDenver International AirportLos Angeles International AirportNewark Liberty International AirportSan Francisco International AirportWashington Dulles International AirportGuam A.B. Wan Pat International AirportAnd one international:Tokyo Narita International AirportAs you can see, these seven major airlines overlap in their hub or major operating city locations nationwide, which can be looked at as a positive thing for frequent travelers. The hub and spoke model offers each airline (even Southwest and JetBlue) a way to replace a high number of half-empty routes with fewer, fuller ones, leading to less delays and a wide variety of connections.Looking for Stocks with Skyrocketing Upside?Zacks has just released a Special Report on the booming investment opportunities of legal marijuana. Ignited by new referendums and legislation, this industry is expected to blast from an already robust $6.7 billion to $20.2 billion in 2021. Early investors stand to make a killing, but you have to be ready to act and know just where to look. See the pot trades we're targeting>>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Southwest Airlines Company (LUV): Free Stock Analysis Report JetBlue Airways Corporation (JBLU): Free Stock Analysis Report Delta Air Lines, Inc. (DAL): Free Stock Analysis Report United Continental Holdings, Inc. (UAL): Free Stock Analysis Report American Airlines Group, Inc. (AAL): Free Stock Analysis Report Alaska Air Group, Inc. (ALK): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research