Nine tubes, bundled
Fazlur Rahman Khan’s structure groups nine square tubes in a three-by-three bundle. The tubes rise to different heights, which gives the tower its stepped silhouette, and the design has been widely used in supertall buildings since.
The world’s tallest building
Topped out on 3 May 1973 and opened that September as the Sears Tower, the building was the tallest in the world until 1998. By its owners’ count it has 110 storeys.
A new name
In 2009 Willis Group Holdings took the naming rights, and on 16 July 2009 the Sears Tower officially became Willis Tower. It remains the tallest building in Chicago.



