In Dallas-Fort Worth, American operates about 81 percent of flights, just ahead of Delta’s 80 percent at Hartsfield-Jackson.
After Atlanta comes Houston’s George Bush Intercontinental Airport, where United operates 76 percent of flights.
Airline hubs benefit from economies of scale, by pooling together millions of passengers and hundreds of flights at a single meeting point to facilitate connections. That allows airlines to gather enough passengers from all over to fill each plane, even to smaller towns, or on overseas flights that require larger aircraft.
In Atlanta, as many as 570,000 possible connections are available in a single day, according to OAG.
Some other cities that have airports heavily dominated by a single airline also have competition in another form: A second airport.
Dallas has Love Field, dominated by Southwest Airlines. Houston has Houston Hobby airport, also dominated by Southwest.
Atlanta has no second commercial airport, though both Delta and Southwest operate at Hartsfield-Jackson. The idea of a second airport sometimes percolates in various forms — most recently via the idea of making Paulding County’s small airport commercial — but has not gone far amid opposition from Delta, the city of Atlanta and residents near potential sites.