AChicago window is a large fixed glass panel flanked by two narrower sashes of the same height, filling a structural bay. The large pane is a single panel of plate glass, and the flanking elements are verticaldouble-hung sash windows with no dividingmuntins. The fenestration was first used by architectWilliam LeBaron Jenney in the 1884Home Insurance Building, and immediately after by several of the Chicago School architecture firms such asHolabird and Root in theirMarquette Building (Chicago),Daniel Burnham'sFisher Building (Chicago), andLouis Sullivan'sCarson Pirie Scott department store. The window design was made possible by advances in glass-making technology and steel structural framing, and became a defining feature of theChicago school style. The design offered both abundant natural light and practical ventilation.[1] Projectingoriel bays are a common variant of the Chicago window, as seen here in theReliance Building (1895) byBurnham and Root.