Louis Sullivan was credited with the phrase "form ever follows function." In the case of the Chicago window, Sullivan's words take on a tangible manifestation. Chicago windows are one of the most prominent (and enduring) features of the (first) Chicago school of architecture, which was prominent in the late 1800s and early 1900s. Like many innovations, Chicago windows are the product of a pressing necessity, or rather two: light and air.
In the late 1800s and early 1900s, modern electricity was not prevalent, and air conditioning was nonexistent. So workers on high floors and inner corridor offices would have been subjected to stifling conditions, not to mention dimness that would make it nearly impossible to see -- except for Chicago windows. Chicago windows, along with steel frame and curtain wall construction, modern elevators and light courts, facilitated the development of the skyscraper.
Chicago windows consist of three sections: a center stationary pane and two smaller panes on either side of the center pane which can be open and closed. Some of the best examples of Chicago windows can be found on an icon of the Chicago school of architecture, the Marquette Building in the heart of Chicago's Loop, completed in 1895. Designed to represent first-class office space of the 1890s, the Marquette Building is splashed out with marble from Italy along with a mosaic designed by the Tiffany company.
Another building that boasts numerous Chicago windows is the Sullivan Center, formerly the downtown home of Carson Pirie Scott, constructed in 1899. In the case of the Sullivan Center, the large Chicago windows were not for the benefit of office workers, but rather intended to show off merchandise inside the structure designed as an upscale shopper's paradise. Sullivan incorporated rows of the iconic windows into the very design of the building, the last major commission of his career.
As the twenty-first century unfolds, there is a renewed emphasis on sustainability in building construction and design, including maximizing natural light, inspired in no small measure by Chicago windows. The "flying carpet" roof of the Modern Wing of the Art Institute of Chicago, completed in 2009, is designed to maximize natural light (while shielding artworks housed inside from harmful UV rays). Maximizing natural light also factors into LEED ratings for new construction and retrofitted buildings.