The Council on Tall buildings and Urban Habitat (CTBUH) was founded at Lehigh University in 1969. Its office remained at Lehigh until October 2003, when it moved to the Illinois Institute of Technology in Chicago, Illinois. Although its stated mission is to study and report "on all aspects of the planning, design, and construction of tall buildings," it is best known to the general public for its compilation of the World's 100 Tallest Building rankings.
The ranking of tall buildings was originally based on a building's height from the sidewalk level of the main entrance to the architectural top of the building. The "architectural top" included towers, spires and pinnacles, but not antennas, masts or flagpoles. In 1996, as Petronas Twin Towers was nearing completion, CTBUH changed its system to identify the tallest building in three additional categories -- the highest occupied floor, the top of the roof, and the top of the pinnacle or antenna.
See also: skyscraper, World's tallest structures
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