The circle, half-circle and quarter-circle are shapes that frequently recur in architecture, particularly in the design of windows and doors. Pure circle windows are used less frequently than the half-circle as an arch at the top of a rectangular window, unless the circle window is part of a nautical design meant to emulate a ship's porthole.
The circle top or arched top window provides an architectural accent as well as admitting additional light to a room. It is frequently segmented into a sunburst pattern for added elegance. A half-circle window can also be used above an entry door to create a transom, allowing additional light into a foyer. There are several different types of circle windows which are used to create visual accents.
From the interior of a home, the addition of a curved shape to a window provides a striking accent to a room. However, many standard window treatments, such as a valence, cannot be used with a circle top or arch top window. Fan-shaped blinds or fabric can be used for the circular portion of the window, but frequently this portion is left open to allow light to enter the room at all times.