Which Chicago building was the world's tallest when completed in 1973?

Sears Tower was the world’s tallest building when it was completed in Chicago in 1973.

The tower was designed by Skidmore, Owings & Merrill, with Bruce Graham as architect and Fazlur Rahman Khan as structural engineer. Its bundled-tube structural system grouped nine square tubes, allowing the building to rise higher while resisting Chicago’s strong winds. The tower reaches 442 metres to its roof and 527 metres including antennas.

Sears, Roebuck & Company was the original anchor tenant, which gave the building its familiar name. In 2009, after a change in naming rights, it became officially known as Willis Tower, although many Chicagoans and visitors still call it Sears Tower.

The building lost the overall height record in 1998 when Kuala Lumpur’s Petronas Towers became taller by the commonly used architectural-height measure. Its engineering system nevertheless became a landmark in skyscraper design and influenced later supertall buildings.