Projecting oriel windows in both halves allow large exposures of glass, giving the building an open appearance despite its mass.

Modern critics have called it a "classic", a "triumph of unified design", and "one of the most exciting aesthetic experiences America's commercial architecture produced".

[4] The Monadnock was commissioned by Boston real estate developers Peter and Shepherd Brooks in the building boom following the Depression of 1873–79.

[13] Between 1881 and 1885, Aldis bought a series of lots in the area on Peter Brooks' behalf, including a 70-by-200-foot (21 by 61 m) site on the corner of Jackson and Dearborn streets.

[18]Early sketches show a 13-story building with Egyptian Revival ornament and a slight flaring at the top, divided visually into five sections with a lotus-blossom decorative motif.

"[21] Objecting at first, Root later threw himself into the design, declaring that the heavy lines of an Egyptian pyramid had captured his imagination and that he would "throw the thing up without a single ornament".

[16][34] Aldis recommended the firm of Holabird & Roche, who had designed the Pontiac Building for Peter Brooks in 1891, to extend the Monadnock south to Van Buren.

[37] The design, for two buildings called the Katahdin and Wachusett (also named for New England mountains), connected them to the north half as a single structure at an estimated cost of $800,000 ($27.1 million in 2023 dollars).

[40] The addition, 17 stories high, preserved the color and vertically massed profile of the original, but was more traditionally ornate in its design, with grander entranceways and more neoclassical touches.

[43] Connected on every floor except the top one and sharing a common basement, each of the four component buildings was equipped with its own heating system, elevators, stairs, and plumbing to facilitate a separate sale if required.

[48] It was the first building in Chicago wired for electricity, and one of the first to be fire-proofed, with hollow fire clay tiles lining the structure so that the metal frame would be protected even if the facing brick were to be destroyed.

[52][h] Aldis filed suit against the "L" in 1901 for $300,000 in damages ($11 million in 2023 dollars), complaining that: [the] means of access to the said building ... had been cut off and the light, air, and view obstructed, and the enjoyment of the property disturbed by the throwing of smoke, dust, cinders, and filth ... by the creating and causing of loud and ominous noises, and by the causing of the ground to shake and vibrate ... said building and premises are greatly damaged.

[16][60][61] The new owners again modernized the interior, installing carpet, fluorescent lights, and new doors, and undertook a major effort to shore up the north wall which had sunk 1.75 inches (44 mm) during construction of the Kluczynski Federal Building across Jackson Street in 1974.

"[64] Donnell, who had studied architecture at Harvard, planned to gut the building and construct a new, modern office complex inside the historic shell.

Failing to obtain financing for the remodeling, he embarked instead on an incremental, "pay as you go"[68] project to restore the Monadnock to its original condition in painstaking detail.

[36] A local firm was found that could reproduce the complicated process of sandblasting and hide glue application used to create the original feather chipped glass.

[85] The original northern half presents a plain, unbroken vertical mass of purple-brown brick, which is contoured to create a gentle curve at the base of the building and an outward flare to form an austere parapet at the top.

[86] The gentle swelling at base and cornice, observed historian Donald Hoffman, "came very close to the bell-shaped column the Egyptians had derived from papyrus".

[91][92] The entire east wall is supported on caissons sunk to the hardpan, installed when the subway Blue Line was dug under Dearborn Street in 1940.

[93] The narrow building allows an external exposure to all of the 300 offices, which pass natural light via double hung outside windows through feather-chipped glass transoms and hallway partitions into the single central corridor.

In the north half, there are two open stairs in the center at the one third points, with perforated risers, white marble treads, and decorative steel railings.

[95] Heavy internal walls at the quarter and halfway points, the arches of which manifest Root's innovative wind bracing, mark the boundaries of the four original buildings.

[102] The district overlaps geographically with the Printer's Row neighborhood, originally the center of Chicago's printing and publishing industry, and now mostly converted to residential housing.

[104][105] Immediately to the west on Jackson Street is the Union League Club of Chicago, founded in 1879 as a civic organization for "upright, law-abiding businessmen".

[110] Contemporary Chicago critics considered the building too radical a departure from Burnham & Root's previous designs and too extreme in its stark simplicity and disregard for prevailing aesthetic norms, calling it an "engineer's house"[111] and a "thoroughly puritanical"[112] example of commercial style.

In the words of French architect Jacques Hermant, "The Monadnock was no longer the result of an artist responding to particular needs with intelligence and drawing from them all of the possible consequences.

New York critic Barr Feree wrote in 1892 that "There are no attempts at facades ... no ornamental appendages, nothing but a succession of windows, frankly stating that the structure is an office building, devoted to business, needing and using every available surface.

"[116] Modern critics have praised the Monadnock as one of the most important exemplars of the Chicago school, along with Louis Sullivan's Carson, Pirie, Scott and Company Building.

[64] It has been called "a triumph of unified design"[117] comparable to Henry Hobson Richardson's Marshall Field's Wholesale Store, and "one of the most exciting aesthetic experiences our commercial architecture has ever produced".

[30][119] The commission went on to note that the "restrained use of brick, soaring massive walls, omission of ornamental forms, unite in a building simple yet majestic.