The Willis Tower (formerly and informally: Sears Tower, its name until 2009) is a 108-story, skyscraper in Chicago. The tower has 108 stories as counted by standard methods, though the building's owners count the main roof as 109 and the mechanical penthouse roof as 110. At completion in 1974, it surpassed the World Trade Center in New York City to become the tallest building in the world, a title that it held for nearly 25 years; it was also the tallest building in the Western Hemisphere for 41 years, until the new One World Trade Center surpassed it in 2013.