United Airlines is one of the largest airlines in the world by fleet size and route network, serving hundreds of destinations across six continents. It operates through a major hub-and-spoke system centered on airports in Chicago, Houston, Denver, Newark, San Francisco, Los Angeles, and Washington D.C. Founded in 1926, United has grown through numerous mergers and acquisitions, most notably its 2010 merger with Continental Airlines, which created one of the world's largest carriers. The combined entity operates under the United Airlines brand and is headquartered in Chicago, Illinois. United Airlines is a founding member of the Star Alliance, the world's largest airline alliance, giving customers access to an extensive global partner network. The carrier has invested significantly in fleet modernization and cabin upgrades in recent years.