The Willis Tower for Android

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 was the tallest building in the Western Hemisphere for 41 years, until the new One World Trade Center surpassed it in 2014. Willis Tower is considered a seminal achievement for architect Fazlur Rahman Khan. It is currently the second-tallest building in the United States and the Western hemisphere and the 16th-tallest in the world. Each year, more than one million people visit its observation deck, the highest in the United States, 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.