tallest building in the world

Previously known as the Sears Tower, the 421m building was completed in 1974 and remained the tallest building in the world for nearly 25 years.Willis Tower (formerly named and still commonly referred to as Sears Tower) is a 108 story, 1,451 foot (442 m) skyscraper in Chicago, Illinois, United States. 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. Willis Tower is the second tallest building in the United States and the eighth tallest freestanding structure in the world. The skyscraper is one of the most popular tourist destinations in Chicago, and over one million people visit its observation deck each year. Named the Sears Tower since its opening, the structure was renamed in 2009 by the Willis Group, as part of their lease on a portion of its space.United Airlines moved its corporate headquarters to Willis Tower from the United Building at 77 West Wacker Drive in August 2012.As of December 2013, United is the Willis Towers largest tenant, with its headquarters and operations center occupying around 20 floors of the tower.