query: What is the worlds tallest building?
passage: List of tallest buildings
The international non-profit organization Council on Tall Buildings and Urban Habitat (CTBUH) was formed in 1969 and announces the title of "The World's Tallest Building" and sets the standards by which buildings are measured. It maintains a list of the 100 tallest completed buildings in the world.[3] The organization currently ranks Burj Khalifa in Dubai as the tallest at 828m (2,717ft).[3] The CTBUH only recognizes buildings that are complete however, and some buildings included within the lists in this article are not considered finished by the CTBUH.
passage: Willis Tower
The Willis Tower, built as and still commonly referred to as the Sears Tower, is a 110-story, 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 remained the tallest building in the Western Hemisphere until the completion of a new building at the rebuilt World Trade Center site in 2014. The building is considered a seminal achievement for architect Fazlur Rahman Khan. The Willis Tower is currently the second-tallest building in the United States and the Western hemisphere – and the 16th-tallest in the world. More than one million people visit its observation deck each year, 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.