The Home Insurance Building in Chicago is often counted as the world's first skyscraper — not for its height but for its load-bearing system. Here the architect William Le Baron Jenney for the first time fully set a building on a metal skeleton.

In traditional masonry the wall carries the load; as the building rises, the lower walls reach an unsupportable thickness — this is the physical limit of how many storeys stone and brick can rise. Jenney's solution was to take the load off the wall and give it to an internal steel (and cast-iron) column-and-beam frame. This frame carries all the load — the floor slabs and the exterior walls; the outer walls are no longer load-bearing but merely a cladding hung on the skeleton (the seed of the curtain-wall idea). The walls thin, the windows grow, and the building can in principle rise far higher.

Though its 10 storeys look low by today's measure, the Home Insurance Building is a proof of principle: what now sets a building's height is no longer the bearing capacity of the wall but the strength of the steel frame and elevator technology. Chicago's rapid rebuilding after the fire and its high land values fed this typology. The Home Insurance Building thus marks the structural birth of the modern skyscraper, establishing the steel-frame principle that made possible the vertical city of the next century.