Original: At the time of its completion in 1973, it was the tallest building in the world, surpassing the World Trade Center towers in New York, and it held this rank for nearly 25 years. The Willis Tower is the tallest building in the United States and the fifth-tallest freestanding structure in the world, as well as the fifth tallest building in the world to the roof.
Simplified: Before the Willis Tower was completed, the tallest building in the world was the World Trade Center.