The Willis Tower, built as and still commonly referred to as the Sears Tower, is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago, Illinois, United States. At completion in 1973, it was the tallest building in the world, a title it held for nearly 25 years and remained the tallest building in the Western Hemisphere until 2014. The building is considered a seminal achievement for its architect Fazlur Rahman Khan. The Willis Tower is the second-tallest building in the United States and the 16th-tallest in the world. The structure was renamed in 2009 by the Willis Group as part of its lease on a portion of the tower's space. (wikipedia.org)