United Airlines, Inc., commonly referred to as United, is a major United States 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.