tracery

Definition

The curving openwork shapes of stone or wood creating a pattern within the upper part of a Gothic window, or an opening of similar character, in the form of mullions which are usually so treated as to be ornamental. By extension, similar patterns applied to walls or panels. We may distinguish between plate and bar tracery; also between geometric, curvilinear (flamboyant) and perpendicular patterns. -- The Illustrated Dictionary of Historic Architecture