Tracery is an architectural device by which windows (or screens, panels, and vaults) are divided into sections of various proportions by stone bars or ribs of moulding. Most commonly, it refers to the stonework elements that support the glass in a window. The term probably derives from the tracing floors on which the complex patterns of windows were laid out in late Gothic architecture. Tracery can also be found on the interior of buildings and the exterior.

There are two main types: plate tracery and the later bar tracery. The evolving style from Romanesque to Gothic architecture and changing features, such as the thinning of lateral walls and enlarging of windows, led to the innovation of tracery. The earliest form of tracery, called plate tracery, began as openings that were pierced from a stone slab. Bar tracery was then implemented, having derived from the plate tracery. However, instead of a slab, the windows were defined by moulded stone mullions, which were lighter and allowed for more openings and intricate designs.

Pointed arch windows of Gothic buildings were initially (late 12th–late 13th centuries) lancet windows, a solution typical of the Early Gothic or First Pointed style and of the Early English Gothic. Plate tracery was the first type of tracery to be developed, emerging in the style called High Gothic. High Gothic is distinguished by the appearance of bar tracery, allowing the construction of much larger window openings, and the development of Curvilinear, Flowing, and Reticulated tracery, ultimately contributing to the Flamboyant style. Late Gothic in most of Europe saw tracery patterns resembling lace develop, while in England Perpendicular Gothic or Third Pointed preferred plainer vertical mullions and transoms. Tracery is practical as well as decorative, because the increasingly large windows of Gothic buildings needed maximum support against the wind.