While designing the Home Insurance Building in 1880s Chicago, William Le Baron Jenney had a brilliant idea: Use metal, rather than heavy stone, to hold up the 10-story structure. At one point, the city of Chicago actually halted construction to investigate the stability of the building and its new approach to assembly, which used a frame to support the building's weight instead of load-bearing walls [source: PBS]. Jenney's plans were sound -- the metal frame was better at supporting weight.
Jenney began construction of the building with an iron frame but switched to steel partway through the project. This would prove to be an incredibly important decision for the history of architecture, as the Home Insurance Building became the template for skyscraper design going forward. Though it was expensive, the use of steel posed a number of advantages aside from stability: Thinner walls made for more spacious interiors and more windows, and fire posed less of a hazard than it did to older masonry structures [source: Chicago History].
There's some controversy about labeling the Home Insurance Building as the first skyscraper. The Equitable Life Assurance Building, completed in 1870 in New York City, used a primitive metal frame in its construction [source: Peterson]. Even if the Home Insurance Building wasn't the very first to use a steel frame, it helped popularize the concept and contributed to the design of skyscrapers as we know them today, and is widely recognized as the first structure of its kind.