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The Willis Tower was designed by architect Bruce Graham and structural engineer Fazlur Rahman Khan of Skidmore, Owings and Merrill (SOM). Graham and Khan designed the building as nine square tubes, clustered in a 3x3 matrix forming a square base with 225-foot (69m) sides. The building's total area is 3,787,200 sq. ft. (351,800 m2). The structure was intended to accommodate 16,500 employees.
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The structure was known as the Sears Tower from its construction until the naming rights were included in a 2009 lease with the Willis Group. Local area residents still refer to the building by its old name. As of April 2018, the building's largest tenant is United Airlines, which occupies around 20 floors. Other major tenants include the building's namesake Willis Towers Watson, and law firms Schiff Hardin and Seyfarth Shaw. Morgan Stanley became the building's fourth-largest tenant in 2017.
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