Tracery is the “Ornamental work of interlaced and branching lines, especially the lacy openwork in a Gothic window” (From The American Heritage® Dictionary of the English Language).
It is likely that tracery evolved from Byzantine windows and pierced marble, and medieval tracery arose from the need to terminate 2 or more mullions, the vertical stone shafts dividing window lights, within the frame of a pointed arch as shown in the grey area in the diagram below.
Early Romanesque church windows were normally small, with a round or segmental (less than a semi-circle) arch at the top. At the start of Gothic architecture, increased understanding of the forces involved in a building and the introduction of buttresses and flying buttresses, windows became larger and wider, with pointed arch designs a can be seen at the Abbey of St. Denis near Paris, glazed in the 1140s by Abbot Suger and from which a small panel is on display in the Burrell Collection showing the Prophet Jeremiah.
These windows were usually divided into 2 or more bays by vertical stone pillars (mullions) which gave the extra support needed by the glass. The earliest tracery decorated the above these bays with small circular, trefoil or quatrefoil lights, which has become known as plate tracery as the design looks as though it has been cut out of a plate of solid stone. The actual construction would have consisted of several pieces of stone. With the trefoil and quatrefoil designs came the introduction of cusps (projecting points) into the arch design.
In the early 13th century, the tracery became slimmer and more varied in shape leading to Bar Tracery, first seen at Reims Cathedral, built in the early 13th century.
Slim stone bars or ribs were used to create a skeleton of geometric shapes, which lead on to flowing, curvilinear and flamboyant designs. Bar tracery allowed a degree of mass production as many blocks could be made using standard templates and could be carved in the workshop when the weather would have been too poor to work on site. The tracery in the Boppard windows is simple but effective, using cusped arches, quatrefoils and subdivided circles with mouchettes (teardrop-shaped designs).
The Islamic designs in Spain at places such as the Great Mosque of Cordoba with its complex tiers of compound arches, had a great influence on Gothic architecture and tracery design. As the Reconquista spread south through Spain, many aspects of the Islamic design of arches were incorporated into Christian designs.
The small lights that were fitted into the holes in the tracery were often designed to complement the imagery, themes and stories displayed in the main panels, with depictions of angels or associated symbols and emblems. Even the tracery could be designed to reflect the content of the glass as in the 14th Century “Tree of Jesse” Window in Dorchester Abbey.
Tracery declined in the 16th century with the Renaissance, but returned in the 19th century with the Gothic Revival.
The name, tracery, probably comes from the ‘tracing floors’ (called épures in French) on which the full size design for the window was drawn, and the blocks of carved stone could be laid down and checked for fit before installation. The floors also allowed for easy communication of ideas between craftsmen and a clear understanding of the finished design. Tracing floors still exist in Wells Cathedral and the Mason’s loft at York Minster.