Airline dominance is highly fragmented. That's because many airlines operate a hub-and-spoke network, concentrating connections at several selected airports spread around the country.
That, in turn, means local travelers have more flights to choose from on the hub carrier, so they fly that airline more often and accumulate frequent flier miles on the hub carrier.
Atlanta-based Delta is one of the largest airlines in the world, with hubs in Atlanta, Minneapolis/St. Paul, Detroit, Salt Lake City, New York, Los Angeles and Seattle.
Yet Delta is not the largest airline at many U.S. airports.
Instead, United and American Airlines operate hubs in places like Chicago; Washington Dulles; Miami; Denver; San Francisco; Newark; Charlotte; Dallas and other airports where they have much larger operations than Delta as a result.
And, Southwest -- a low-cost carrier that doesn’t operate a traditional hub-and-spoke network and instead has focus cities across the country -- carries more domestic passengers than any of the big legacy carriers.
United, American and Delta are more expansive than Southwest in other ways -- including a larger international presence and in turn carrying more passengers on longer flights.
But in different cities around the country, the first airline passengers think of often depends on the regional dominance of carriers.