The structure was originally built in the shape of a blunt-ended U. It measures 352 feet north to south, 134 feet east to west, and 90 feet from the sidewalk to the balustrade. The three-foot thick walls are made of fine grade Bedford (Indiana) limestone on a granite base. From the outside the building appears to be three stories but inside it is actually five stories.
The north and south sides of the building have different architectural elements. Entering from Randolph Street, the north side, the architecture is Greek-inspired with strong angular structures and military-influenced decorations. The three doorways share a massive porch with Doric columns set in pairs.
From the curving marble staircase an outdoor garden and sculpture can be viewed. At the top of the stairway is the 45-foot by 50-foot Grand Army of the Republic (G.A.R.) Rotunda. The ceiling is embossed with plaster carvings of swords, shields, helmets, and flags. This ornamental heraldry serves to remind viewers of the loss that comes with war.
The 40-foot-diameter stained-glass dome in shades of tan, beige, and ochre is now lighted electrically. It was originally illuminated by sunlight. The stained glass was made by Healy & Millet of Chicago. It is held by cast-iron ribbing, manufactured by the Winslow Brothers of Chicago. A floor inset with glass blocks originally provided natural light from the dome to the first floor below.
The immense G.A.R. Memorial Hall is just beyond the Rotunda. It measures 53-feet long, 96-feet wide, and 33-feet high. Leased to the Grand Army Hall and Memorial Association between 1898 and 1948, it was a meeting place for members of the G.A.R. Today, the collection of Civil War artifacts once displayed there is now preserved at the Harold Washington Library Center. It is used for ceremonial and artistic purposes, including weddings.
This room is a somber and richly decorated memorial to the soldiers of the Civil War. The sedate Vermont (Verdé) marble walls bear the names of 30 Civil War battles including: Shilo, Antietam, Gettysburg, Cedar Creek, Ft. Sumter. The coffered ceilings are encrusted with dragons, fruit, starts, and other designs. Adjacent to Memorial Hall is the Claudia Cassidy Theater, originally a flat-floored G.A.R. meeting room.
Entering from Washington Street, the south side, the architecture is Roman-inspired. This entrance has Roman arches and exuberant ornamentation. There are three pairs of glass doors with decorative elements. A 34-foot-long elliptical arch of white marble, decorated with glass tesserae, sparkles with the names of great thinkers of the past, including Cicero, Plato, and Livy. The lobby (45 feet deep and 53 feet wide) is decorated in rare marbles. The white marble is Italian Carrara, from the same source as the marble used by Michelangelo for his sculpture. The dark green marble is Irish Connemara. Fine hardwoods, stained glass, and polished bronze are also used lavishly throughout.
The Cosmati work throughout the interior is a technique in which marble is inlaid with a variety of materials, including lustrous Favrile glass, colored stone, mother-of-pearl, gold leaf, and mosaic. This technique makes walls appear jewel-like. Due to Chicago’s sooty air, the choice of glass and marble was practical as well as aesthetic because these materials last indefinitely and can be easily cleaned.
The grand staircase of white Carrara contains mosaics designed by Robert C. Spencer, Jr. of Shepley Rutan and Coolidge. The mosaics were executed in the Tiffany Studios in New York by J. A. Holzer. Above the third floor the staircase is decorated with less elaborate Italian and American marbles and mosaics.
On the third floor the staircase opens into the elegant Preston Bradley Hall, which spans the width of the building. Originally, this was the general delivery room where people received the books they requested.
The magnificent translucent dome, 38 feet in diameter and made of Tiffany Favrile glass, is cut in the shape of fish scales. At the top of the dome are the signs of the zodiac. Now lighted electrically, it was originally illuminated by sunlight. At the base of the dome is a quotation from the British author Joseph Addison. The dome glass, lighting fixtures, wall sconces and chandeliers were made by the Tiffany Glass and Decorating Company of New York. The supporting frame was constructed by the Chicago Ornamental Iron Company. On the east and west sides of the hall are quotations in Greek, Chinese, Arabic, Egyptian hieroglyphics, Hebrew, Italian, German, French, Latin, and Spanish. Black ornamented boxes in the corners of the room were once elevators used for book delivery.