United Airlines Holdings, Inc. (formerly known as United Continental Holdings, Inc., UAL Corporation, Allegis Corporation and founded originally as UAL, Inc.) is a publicly traded airline holding company headquartered in the Willis Tower in Chicago. UAH owns and operates United Airlines, Inc. UAL Corporation agreed to change its name to United Continental Holdings in May 2010, when an agreement was reached between United and Continental Airlines where United would acquire Continental. To effect the acquisition, Continental shareholders received 1.05 shares of UAL stock for each Continental share; at the time of closing, it was estimated that United shareholders owned 55% of the merged entity and Continental shareholders owned 45%. The company or its subsidiary airlines also have several other subsidiaries. Once completely combined, United became the world's largest airline, as measured by revenue passenger miles. United is a founding member of the Star Alliance.