Chicago School (fl. 1880-1910) architecture refers to the proto-Modernist expression of early skyscrapers in Chicago between 1880 and 1900, which began to emphasize their steel frame on the facade through vertical piers and large windows. Although we would consider them ornamented today, especially the work of Louis Sullivan, Chicago School buildings feature much less decoration than other buildings in the revival styles popular at the time. The style influenced architects across the United States and Canada, also spawning the smaller-scale Commercial style for less populated cities.
The earliest building considered to be a Chicago School design is the 1879 Leiter Building I (demolished 1972), which was the first to combine all four elements of early skyscrapers--a tall height, an iron frame, terra-cotta fireproofing, and elevators. Its highly restrained ornamentation and function-focused expression and layout was very innovative in a time of lavishly decorated Victorian buildings.
The 1883 Montauk Building, a 10-story building with steel construction, was the first building called a "skyscraper" and was constructed through the evenings and in winter, a first for the time. The Home Insurance Building, although expressed in a classical manner much different from other Chicago School buildings, is considered the first true skyscraper due to its load-bearing steel frame construction.
Chicago School buildings are characterized by their unique design that would eventually lead to orthodox Modern architecture in the United States. They express their steel structure through large window bays and uninterrupted vertical piers, indicating the exterior walls as curtain walls that do not carry the building's load. In terms of the entire composition, many use a "base-shaft-capital" method to articulate the building. Massing is very rectangular, lacking any setbacks or follies such as towers/turrets. Facades are built of masonry or terra-cotta. Ornamentation is typically minimal due to the large windows, usually confined to the base and cornice if present at all. Even the windows themselves are unique, known as "Chicago windows," which surround a central fixed pane with two smaller double-hung windows. These can be flat or bay windows.
Construction of these early skyscrapers continued into the 1890s and the early 20th century, though after the World's Columbian Exposition of 1893, Beaux-Arts and other classical revivals became more popular for skyscraper construction. The last true Chicago School buildings were built around 1910.
Wikipedia asserts the existence of a "Second Chicago School" consisting of International-style work by Mies van der Rohe and Skidmore, Owings, & Merrill, but most of these buildings conform to the International style and many similar designs were built across the country, not just in Chicago.