was to make the central light
the highest, and to graduate the height of the others. It after a time
became customary to render the opening more ornamental by adding
pointed projections called cusps. By these the shape of the head of
the opening was turned into a form resembling a trefoil leaf.
Sometimes two cusps were added on each side. The head is, in the
former case, said to be trefoiled--in the latter, cinqfoiled.

[Illustration: FIG. 17.--TWO-LIGHT WINDOW. (13TH CENTURY.)]

[Illustration: 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).

[Illustration: 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 w