It used to be the Sears Tower, but in 2009, London-based insurance broker Willis Group Holdings acquired the naming rights. No matter what you call it (everybody still refers to it as Sears Tower), it's an architectural icon. When it was built in 1973 it was the tallest building in the world. It's now 14th-tallest, but offers unrivaled views of the Windy City, and sees more than a million visitors each year.
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Also you will enter a bit more rapidely at the visiting location