Completed in 1974 as the Sears Tower, this 110-story office tower was built for Sears, Roebuck and Co., which kept its headquarters here until 1994. When it opened in 1973 it became the tallest building in the world, a title it held for nearly 25 years, until the Petronas Towers rose in 1998.
Its height was made possible by the "bundled tube," a structural system devised by architect Bruce Graham and engineer Fazlur Rahman Khan of Skidmore, Owings & Merrill. Nine square steel tubes are clustered three by three. They rise together, then drop away at different floors, giving the tower its stepped silhouette while each tube braces the others against the wind.
The building was renamed Willis Tower in 2009. The Skydeck on the 103rd floor, about 1,353 feet up, includes the Ledge: glass boxes that project from the facade, letting visitors look straight down to the street.
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