The Willis Tower (Sears Tower)

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The Willis Tower (formerly named, and still commonly referred to as, the Sears Tower) is a 108-story, 1,451-foot (442 m) skyscraper in Chicago, Illinois.[4] At the time of its completion in 1973, it was the tallest building in the world, surpassing the World Trade Center towers in New York, and it held this rank for nearly 25 years. The Willis Tower is the tallest building in the United States and the seventh-tallest freestanding structure in the world.

Although Sears' naming rights expired in 2003, the building continued to be called the Sears Tower for several years. In March 2009, London-based insurance broker Willis Group Holdings agreed to lease a portion of the building, and obtained the building's naming rights. On July 16, 2009, the building was officially renamed the Willis Tower.

http://en.wikipedia.org/wiki/Willis_Tower