Southwest Airlines Co. is a major U.S. passenger airline operating an all-Boeing 737 fleet, primarily serving short-haul, point-to-point routes across an extensive domestic network, with additional service to Mexico, the Caribbean, and Central America. The airline carries more domestic passengers than any other U.S. carrier. Historically known for a single open-seating cabin, free checked bags, and no assigned seats, Southwest has recently introduced assigned seating, extra-legroom rows, and multiple fare tiers, including a basic economy option, to compete more directly with other airlines. Revenue comes mainly from passenger ticket sales, supplemented by ancillary fees, loyalty program partnerships through its Rapid Rewards program, and cargo and other services. Southwest targets both leisure and business travelers, with a significant presence at secondary and high-traffic airports throughout the United States.