What features made the Chicago windows of the Schlesinger and Mayer department store distinctive?

The distinctive feature of the Chicago windows used in the Schlesinger & Mayer department store (now the Carson Pirie Scott building) is their rectangular design, which consists of a large, fixed central pane flanked by two smaller, movable sash windows.1

These windows are a notable element of the building's upper floors, which are characterized by an elegant simplicity that contrasts with the lavish, cast-iron ornamentation found on the first two levels. While the department store's overall design emphasizes horizontal lines—a departure from the vertical focus of Sullivan's earlier office buildings—the Chicago windows serve as a functional and aesthetic component of the facade's grid-like structure.1