In architecture, tracery usually refers to the stonework elements supporting the glass in a Gothic window, though it may also appear simply as a design element on other surfaces, in which case it it called blind. Screens may also feature openwork tracery with no glass involved. There are two major types of this design element: plate and bar. Like many elements of Gothic architecture, it shows a distinct influence from contemporary Islamic architecture.
Gothic architecture, a style beginning in the 12th century, when it was typically used in cathedrals, featured, among many other architectural elements, windows with a pointed arch on top. As window openings became larger, multiple pieces of glass were used per window. The most popular window design in early Gothic architecture consisted of two side-by-side lancets, or tall, thin windows with pointed arches, surmounted by a round or trefoil opening, often within a blind pointed arch giving the entire window bay the same shape as each lancet. A trefoil opening is a pattern of three overlapping circles. The design of the windows in the top of the arch, above the lancets, could be rather elaborate and made use of tracery.
The earliest form of tracery was the plate variety, which has the appearance of holes being cut out of a large piece of masonry, though in actuality, the design is constructed of many separate pieces of stone. Bar tracery, which developed in the early 13th century, resembles thin coils of stone bent into complex patterns. The mullions, or narrow stone pieces used to form the window designs, allowed for more intricate and delicate patterns, increasing the possible space for glass and thereby the amount of light let through the window. Bar tracery allowed not only for more intricate patterns, but also for mullions to be mass-produced according to templates.
The ability to mass-produce tracery components led to increased applications for the technique. Blind tracery became used on walls, while openwork design was often used over windows. The design could either match that of the window or complement it to produce an even more elaborate pattern.