Willis Tower (originally named the Sears Tower) is a 110-story skyscraper built in 1973. It was the tallest building in the world for nearly 25 years. Originally inspired by a pack of cigarettes, the building used the tubular system pioneered on the John Hancock Center. It was designed as nine tubes (with square footprints 75 feet on a side) clustered in a 3 x 3 matrix. The tubes rise together for the first 50 stories and terminate at varying heights above that with just two of the tubes rising the entire height of the building.