Southwest Airlines is one of the major airlines of the United States and the world's largest low-cost carrier. It is headquartered in Dallas, Texas and has scheduled service to 121 destinations in the United States and 10 additional countries. Southwest carries more domestic passengers than any other United States airline. The airline was established on March 15, 1967, by Herb Kelleher and Rollin King as Air Southwest Co. and adopted its current name, Southwest Airlines, in 1971, when it began operating as an intrastate airline wholly within the state of Texas, first flying between Dallas, Houston, and San Antonio. It began regional interstate service in 1979, expanding nationwide in the following decades. Southwest currently serves airports in 45 states and multiple Central American destinations. Southwest's business model is distinct from other US airlines as it uses a rolling hub and point-to-point network and allows free checked baggage. It exclusively uses Boeing 737 jets in its fleet.