company, United Airlines Holdings, Inc., which is a Delaware corporation, on the New York Stock Exchange , with a market capitalization of over US$21 billion as of January 2018. United's operating revenues and operating expenses comprise nearly 100% of the holding company's revenues and operating expenses. Headquarters and other facilities United Airlines headquarters are located at the Willis Tower, 233 South Wacker Drive, Chicago, Illinois. In 2007, United had moved its headquarters from Elk Grove Township, a suburb of Chicago, to 77 West Wacker Drive in the Chicago Loop after receiving US$5.5 million in incentives from the City of Chicago. Then in 2010, United accepted the City of Chicago's offer of US$35 million in incentives, including a US$10 million grant, for United to move its remaining 2,500 employees out of Elk Grove Township to the Willis Tower (formerly the Sears Tower) in the Chicago Loop. On May 31, 2012, United opened its operations center, which occupied twelve floors there. In 2019 United renewed its lease at Willis Tower, originally ending in 2028 and now set to expire in 2033, and plans to construct a roof deck and a dining hall on the fourth floor. The former headquarters campus at Elk Grove Township was gradually annexed into the Village of Mount Prospect, and serves as an IT operations facility, with a new data center constructed in 2013. United maintains a large presence in downtown Houston, having leased of space (seven floors) for occupancy in 2017. United has training facilities for its flight crews in Denver and Houston, a major aircraft maintenance center in San Francisco, and call centers in Houston and Chicago. On September 24, 2020, United Airlines announced that it will roll out a new COVID-19 testing program for passengers from October 15 that year. Initially, testing was only available for passengers traveling to Hawaii from San Francisco International Airport. Corporate identity Brand image The pre-merger United logo, commonly nicknamed the "tulip", was developed in the early 1970s by the designer Saul Bass as part of a new brand image. The logo represented the airline's monogram as well as a modernized version of the airline's shield logo which had been adopted in the 1930s, but fell out of use by the late 1960s. The ribbon-like rendering has also been said to symbolize the motion of flight. Marketing themes United's earliest slogan, "The Main Line Airway", emphasized its signature New York-Chicago-San