The northern half is built with pre-skyscraper technology: supported not by an internal skeleton but by brick walls. For this reason the walls are abnormally thick (six feet!) at the base.

At one time the building was divided into 4 sections, each named after a different New England mountain: Monadnock, Kearsarge, Katahdin, and Wachusett.

The Monadnock was architect John Root's last building. Holabird and Roche designed the 17-story southern addition in 1893.

The spread foundations are so wide that they extend 11 feet beyond the building's lot under the surrounding streets.

For the north wing, architect Root explored using variated facade colors and Egyptian ornament. The developer insisted on no ornament, but Egyptian forms survive in the massing and the flared parapet.

The Monadnock was the first high-rise building to use portal bracing, although this was not the more distinctive arched portal bracing used in the neighboring Old Colony Building.

Early drawings show this as a 12-story building, but a threatened change in zoning laws prompted the developers to get a permit for 16 floors while still possible.

The southern addition has one extra floor, which is even with the windowless attic of the north wing.

An elaborate iron staircase runs up the middle of the north building, right through the lobby and the office floor hallways.

Chief developer Peter Brooks originally ruled out any projecting bay windows, but he was persuaded of their financial benefit by his Chicago agent Owen Aldis.