Airlines use certain airports as gateways to multiple destinations; these are called hubs. An airline hub is an airport where a particular airline offers a myriad to direct flights allowing passengers to transfer to connecting flights to their final destination. Oftentimes, travelers are headed to a destination not served by a direct flight from their departing airport. The airline they choose to fly with takes passengers to their gateway so they can transfer to their connecting flight and begin the final leg of their trip.
Airlines locate their major connection points in cities all over the world; and many airlines use more than one. Airline route maps are available on each flight, typically in the in-flight magazine on-board. These maps allow passengers to see where the airline on which they are flying will land between each flight.
It comes as no surprise that most major airline hubs are located in the biggest, busiest airports in the world. In North America, major connectors are located all over the continent. BothSouthwest and Delta Air Line call the Hartsfield-Jackson Atlanta International Airport in Georgia their main base of operations. American Airlines uses the Dallas-Fort Worth International Airport in Texas as their main base. United Airlines' main connectors are in Denver, Chicago Ohare, and George Bush Intercontinental Airport. The Phoenix Sky Harbor International Airport in Arizona is the main hub for US Airways, which also has a hub in Charlotte. The JFK airport in New York City is the major base for Jet Blue Airlines and both United and Frontier Airlines call the Denver International Airport their main airline hub. Air Canada is also a North American airline and its hub airport is the Toronto Pearson International Airport in Canada.
Of course, there are plenty of other major airlines in countries other than North America that have their own base cities. British Airways uses the London Heathrow Airport as its main hub. Lufthansa operates principally from Flughafen Frankfurt am Main in Germany, and Aer Lingus' main base is the Belfast International Airport in the United Kingdom. Air France KLM airline's main hub is at the Paris-Charles de Gaulle Airport, and Qantas Airlines calls the Sydney Airport in Australia its main location.
The Honolulu International Airport in Hawaii, for instance, is the major hub for Hawaiian Airlines. Alaska Airlines has three major airport bases located in Anchorage, Seattle and Portland.
Anytime a traveler departs from an airport to a location that does not have a direct flight, they have a layover in airline connector city. The airline hub cities where travelers make a stop depends on the airline and the destination to which they are departing.