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 purpose of the device is practical as well as decorative, because the increasingly large windows of Gothic buildings needed maximum support against the wind. The term probably derives from the tracing floors on which the complex patterns of windows were laid out in late Gothic architecture. Tracery can be found on the exterior of buildings as well as the interior.

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.

Plate tracery, in which lights were pierced in a thin wall of ashlar, allowed a window arch to have more than one light – typically two side by side and separated by flat stone spandrels. The spandrels were then sculpted into figures like a roundel or a quatrefoil. Plate tracery reached the height of its sophistication with the 12th-century windows of Chartres Cathedral and in the "Dean's Eye" rose window at Lincoln Cathedral.

The earliest form of window tracery, typical of Gothic architecture before the early 13th century, is known as plate tracery because the individual lights (the glazed openings in the window) have the appearance of being cut out of a flat plate of masonry. Romanesque church windows were normally quite small, somewhat taller than wide and with a simple round-headed ('segmental') arch at the top. From around the 1140s, the pointed arch Gothic window (employed by Abbot Suger for the redesign of the choir at St Denis) started to take over.

As the buttressing systems of early Gothic architecture reduced the structural need for broad expanses of thick walls, window openings grew progressively larger and instead of having just one very large window per bay division (which would create problems with supporting the glass), the typical early-Gothic 'twin lancet plus oculus' form of plate tracery developed. This consists of two (sometimes three) tall thin lights topped with pointed arches, with a round or trefoil opening placed above them, often contained within a blind arch which gives the whole assemblage a pointed lancet shape (see the example from Soissons Cathedral). With this type of design, the spandrels (i.e. the spaces between the tops of the lancet windows and the oculus) are just blank wall. The practicalities of building window tracery in this way severely limited the complexity of designs that could be produced and although plate tracery designs evolved over the course of the 12th and early 13th centuries, in practice, the only real variation was in the number and size of lancets and in the trefoils, quatrefoils and oculi used to fill the spaces above them.

The rose windows of early- and high-Gothic cathedrals, such as the example in the north transept of Laon Cathedral (1170s) or the west facade at Chartres (c. 1210), also employed plate tracery. This greatly limited the overall amount of light admitted to the interior by these windows, as well as restricting the complexity of patterns that could be created.

At the beginning of the 13th century, plate tracery was superseded by bar tracery. Bar tracery divides the large lights from one another with moulded mullions. Bar tracery, an important decorative element of Gothic styles, appeared first at Reims and was employed in England around 1240. After 1220, master builders in England had begun to treat the window openings as a series of openings divided by thin stone bars, while before 1230 the apse chapels of Reims Cathedral were decorated with bar tracery with cusped circles (with bars radiating from the centre). Bar tracery became common after c. 1240, with increasing complexity and decreasing weight. The lines of the mullions continued beyond the tops of the window lights and subdivided the open spandrels above the lights into a variety of decorative shapes. Rayonnant style (c. 1230–c. 1350) was enabled by the development of bar tracery in Continental Europe and is named for the radiation of lights around a central point in circular rose windows. Rayonnant also deployed mouldings of two different types in tracery, where earlier styles had used moulding of a single size, with different sizes of mullions. The rose windows of Notre-Dame de Paris (c. 1270) are typical.