When it was completed in 1893, the Monadnock was the world's largest office building.
"The two halves of the building are similar in scale and color, but quite different in style. The north half is often called a fountainhead of modern architecture because of its total absence of exterior ornament. Root evidently felt that all that was needed here was graceful form for the structure itself. The south half of the building, on the other hand, is a masterful early application of classical architectural principles to the design of a tall building.
The Monadnock also marks a historic transition in the development of structural methods. Most of the buildings that preceded it were supported by their outside walls. The north half of the Monadnock is probably the tallest building ever built that is supported primarily by brick walls. At ground level, those walls are six feet thick. Half of the south half of the building is built the same way, but the south quarter of the building is supported entirely by a steel frame, as were most of the tall buildings that followed it. Today, this is called "curtain wall" construction: the facade doesn't support the building, it's just a "curtain" to keep out the elements."