Hang a building's floors on an internal frame of steel rather than stacking them on load-bearing walls, and the structure can rise as high as the frame holds. Add the safety elevator and the upper floors become prime real estate. That combination, pioneered in Chicago after the 1871 fire, produced the skyscraper — a form usually dated to the Home Insurance Building of 1885. It concentrated offices and capital onto expensive urban plots, created the modern skyline, and became architecture's clearest symbol of corporate ambition. The race for the tallest now runs through Dubai and Shanghai.
Stack stone on stone and the wall at the bottom must grow thicker the higher you build, until the lower floors are almost solid masonry with no room left to rent. That ceiling on height held for the whole history of building. Then, in the Chicago of the 1880s, engineers hung a building's floors and walls on an inner cage of steel instead, so the frame carried the weight and the outer walls became a thin skin that could be cut away into glass. A second machine made the new height livable: the safety elevator, which would not fall even if its cable snapped. Together they let the city stop spreading outward and start climbing, piling floor upon floor into rentable air. The tower that resulted became the boldest thing humans routinely build and the clearest sign in the world of money and ambition. It was also raised by men who walked steel beams a thousand feet up, with nothing beneath them but the street.