United Airlines, Inc., commonly referred to as United, is a major U.S. airline headquartered in Chicago, Illinois. It is the world's third-largest airline when measured by revenue, after American Airlines and Delta Air Lines. United operates a large domestic and international route network, with an extensive presence in the Asia-Pacific region. United is a founding member of the Star Alliance, the world's largest airline alliance with a total of 28 member airlines. Regional service is operated by independent carriers under the brand name United Express. Its main competitors are American Airlines, Delta Air Lines, Alaska Airlines, and Southwest Airlines.