Louis Sullivan. Wainwright Building, St. Louis MO. 1890-91. Architects can’t build out, so they build up! Use of steel for first time, previously buildings were made of marble, masonry, brick, even iron. But iron wasn’t ideal because it rusted and it burned. Steel was more durable and lighter, able to support a lot of weight. Development of elevators around this time, too. Steel skeleton, then masonry is laid over it, kind of like a shell. This is called a curtain wall, which gives ultimate form. Chicago School was group of architects loosely based around Chicago that popularized sky scrapers as a kind of building form.