United Airlines, also known as United Air Lines, Inc. is a major airline of the United States. It is a subsidiary of UAL Corporation with corporate offices in Chicago, Illinois at 77 West Wacker Drive, and its operations base in nearby Elk Grove Township. United's largest hub is O'Hare International Airport, where it has 650 daily departures. United also has hubs in Denver International Airport, Washington Dulles International Airport, San Francisco International Airport, and Los Angeles International Airport. Its largest maintenance facility is at San Francisco International Airport.
As of July 31, 2006, United is the world's second-largest airline by revenue-passenger-miles (behind American Airlines), third-largest by total operating revenues (behind Air France-KLM and American Airlines), and fourth-largest by total passengers transported (behind American Airlines, Delta Air Lines and Southwest Airlines).