Built in 1973, the Sears Tower is a classic example of modern architecture. But its towering shape and smooth exterior have a long history. They are a part of the development of modern architectural design. Throughout the nineteenth century, thousands of Americans moved from rural areas to cities such as Chicago. Many came to work in factories. Some found jobs in offices, stores, hotels, or restaurants. As the cities grew, neighborhoods became crowded. Open space was hard to come by. Architects realized that new buildings would have to rise up, not spread out.
In France in the late 1800s, Gustave Eiffel had built the tallest tower in the world out of wrought iron. The iron was strong, and it weighed much less than stone. In the United States, architects and engineers were experimenting with iron frames. Buildings could rise much higher if they were constructed around an iron frame rather than being built of solid stone walls. Soon iron framing was replaced by steel. Steel is iron combined with other chemical elements such as carbon. The added chemicals make steel harder and stronger. Steel can also be heated and pulled, stretched, or shaped without breaking.