the top of the arcading. Windows Light, and therefore the window, was a central feature of Rayonnant architecture; Rayonnant windows were larger, more numerous, and more ornate than in earlier styles. They also frequently had clear or grisaille glass, brightening up the interior. The shadows and darkness of early Gothic cathedrals, with their small windows and deep, rich colors such as Chartres blue, was replaced by a brightly lit space with a full spectrum of coloured light. Intermediate levels of the walls, such as the Triforium, were given windows. At the high level of the clerestory, rows of lancet windows appeared, often topped with tri-lobed or four-part windows and a type of miniature rose windows, called an oculus. This was made possible at Notre-Dame by the construction of taller and longer kind of flying buttress that made a double leap to support the higher sections of the walls. There was also a fundamental change in the tracery, or ornamental designs, within windows. Early Gothic windows often used plate-tracery (in which the window openings look as if they have been punched out of a flat stone plate. This was replaced by the more delicate bar-tracery in which the stone ribs separating the glass panels are made of narrow carved mouldings, with rounded inner and outer profiles. The elaborate designs of the spokes of the rose windows, radiating outward, gave the name to the Rayonnant style. Bar-tracery probably made its first appearance in the clerestory windows at Reims Cathedral and quickly spread across Europe. A notable architectural innovation that emerged as part of the Rayonnant style in France was the use of glazed triforia. Traditionally, the triforium of an Early or High Gothic cathedral was a dark horizontal band, usually housing a narrow passageway, that separated the top of the arcade from the clerestory. Although it made the interior darker, it was a necessary feature to accommodate the sloping lean-to roofs over the side aisles and chapels. The Rayonnant solution to this, as employed to brilliant effect in the 1230s nave of the Abbey Church of St Denis, was to use double-pitched roofs over the aisles, with hidden gutters to drain off the rainwater. This meant the outer wall of the triforioum passage could now be glazed, and the inner wall reduced to slender bar tracery. Architects also began to emphasise the linkage between triforium and clerestory by extending the central mullions