Tracery
Tracery is an architectural device by which windows 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 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
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 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 arch at the top. From around the 1140s, the pointed arch Gothic window 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, the typical early-Gothic 'twin lancet plus oculus' form of plate tracery developed. This consists of two 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. With this type of design, the spandrels 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 or the west facade at Chartres, 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.
Bar tracery
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. 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 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 are typical.The earliest bar tracery designs were made for the aisle windows at Reims Cathedral around 1215. The Reims windows still used the same 'two lancets plus oculus' pattern, but now the glass panels were held between narrow stone mullions made up of carefully shaped lengths of masonry quite distinct from the wall surrounding them. These mullions were much more slender than the corresponding elements in plate-tracery windows and crucially, the previously solid wall areas such as the spandrels could also now be glazed, greatly increasing the amount of light admitted.
The cross-section of each mullion or tracery bar was important both for the structural integrity of the window and for the visual effect. As can be seen in Viollet-le-Duc's diagram there was normally a roll-moulding on both the inside and outside of the windows, which made the mullions appear even more slender than they actually were. The shoulder marked 'B' on the diagram is the glazing slot, into which the metal frame of the window glass is mounted. Unlike with plate tracery, where each stone had to be individually shaped, the elements of bar tracery could be mass-produced to standard templates in the mason's yard – work that could continue even when it was too cold for lime mortar to set. The technical aspects of the windows at Reims clearly fascinated Villard de Honnecourt, who visited the construction site, probably in the 1220s, and made a detailed sketch of the various templates, using a key to show how they fitted into the different parts of the window.
Image:20130406 Ely Cathedral 01.jpg|thumb|upright|Geometrical bar tracery, Ely Cathedral, Lady Chapel, west window|alt=
Geometrical tracery
The early phase of Middle Pointed style is characterized by Geometrical tracery – simple bar tracery forming patterns of foiled arches and circles interspersed with triangular lights. The mullions of Geometrical style typically had capitals with curved bars emerging from them. Intersecting bar tracery deployed mullions without capitals which branched off equidistant to the window-head. The window-heads themselves were formed of equal curves forming a pointed arch and the tracery bars were curved by drawing curves with differing radii from the same centres as the window-heads. The mullions were in consequence branched into Y-shaped designs further ornamented with cusps. The intersecting branches produced an array of lozenge-shaped lights in between numerous lancet arched lights. Y-tracery was often employed in two-light windows c. 1300.Geometrical tracery is identified by the circular openings at the head of the arch of the window. A common composition is three lights beneath two circles and a third at the point of the arch; such an example can be seen along the aisle at Lincoln Cathedral Also at Lincoln Cathedral, the east window is an expanded version of this idea with two interior arches, a total of eight lower lights, four small circular lights topped with two larger circles to fill out the interior arches, and finally above all one large circular shape filled with seven smaller circular lights. Geometrical tracery, in its early stages, had a rule of equilateral law, where the tracery design follows the shape of the arch in an equilateral manner. Additional decorative elements can be implemented, such as foliation or the "spherical triangle". The use of spherical triangles is a later adaption and likely reflects religious significance.
Curvilinear (flowing) tracery
Second Pointed saw Intersecting tracery elaborated with ogees, creating a complex reticular design known as Reticulated tracery. Second Pointed architecture deployed tracery in highly decorated fashion known as Curvilinear and Flowing. These types of bar tracery were developed further throughout Europe in the 15th century into the Flamboyant style, named for the characteristic flame-shaped spaces between the tracery bars. These shapes are known as daggers, fish-bladders, or mouchettes.Starting in the late 13th century and at the beginning of the 14th century, tracery took on more fluid characteristics. A common shape used in curvilinear tracery was that of the ogee, which was too weak for structural application and was instead used as a decorative element. The use of the ogee in curvilinear tracery can be seen in the west window of St Mary's parish church in Cottingham, East Riding of Yorkshire.