Fig. 17.—Two-light Window. (13th Century.)
Fig. 18.—Geometrical Tracery. (14th Century.)
When two windows were placed close together it began to be customary to include them under one outer arch, and after a time to pierce the solid head between them with a circle, which frequently was cusped, forming often a quatrefoil (Fig. [17]). This completed the idea of a group, and was rapidly followed by ornamental treatment. Three, four, five, or more windows (which in such a position are often termed lights) were often placed under one arch, the head of which was filled by a more or less rich group of circles; mouldings were added, and thus rose the system of decoration for window-heads known as tracery. So long as the tracery preserves the simple character of piercings through a flat stone, filling the space between the window heads, it is known as plate tracery. The thinning down of the blank space to a comparatively narrow surface went on, and by and by the use of mouldings caused that plain surface to resemble bars of stone bent into a circular form: this was called bar tracery, and it is in this form that tracery is chiefly employed in England (Fig. [18]). Westminster Abbey is full of exquisite examples of E. E. window-tracery (temp. Henry III.); as, for example, in the windows of the choir, the great circular windows (technically termed rose-windows) at the ends of the transepts, the windows of the chapter-house. Last, but not least, the splendid arcade which forms the triforium is filled with tracery similar in every respect to the best window tracery of the period (Fig. [19]).
Fig. 19.—The Triforium Arcade, Westminster Abbey. (1269.)
In the decorated style of the fourteenth century tracery was developed till it reached a great pitch of perfection and intricacy. In the earlier half of the century none save regular geometrical forms, made up of circles and segments of circles, occur; in other words, the whole design of the most elaborate window could be drawn with the compasses, and a curve of contrary flexure rarely occurred. In the latest half of that period flowing lines are introduced into the tracery, and very much alter its character (Fig. [20]). The cusping throughout is bolder than in the E. E. period.