The Willis Tower (colloquial: Sears Tower, its name for 36 years) is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago, Illinois. At completion in 1973, it surpassed the World Trade Center in New York to become the tallest building in the world, a title it held for nearly 25 years; it was the tallest building in the Western Hemisphere for 41 years, until the new One World Trade Center surpassed it in 2014. While it held the title of "Tallest Office Building" until 2014, it lost the title of "Tallest Man-Made Structure" after only 3 years. The CN Tower took the title in 1976 which serves as a communication tower in Toronto. Willis Tower is considered a seminal achievement for architect Fazlur Rahman Khan. It is currently the third-tallest building in the United States and the Western hemisphere – and the 23rd-tallest in the world. Each year, more than one million people visit its observation deck, the highest in the United States, making it one of Chicago's most popular tourist destinations. The structure was renamed in 2009 by the Willis Group as a term of its lease. As of April 2018, the building's largest tenant is United Airlines, which moved its corporate headquarters from 77 West Wacker Drive (then the United Building) in 2012, occupying around 20 floors. Other major tenants include the building's namesake Willis Group and law firms Schiff Hardin and Seyfarth Shaw. Morgan Stanley plans to move to the building in 2019 and become its fourth-largest tenant by 2020.