In architecture, bars or ribs used decoratively in windows, especially the ornamental openwork in Gothic windows. In the earliest phase, two or three narrow, arched windows were placed close together under a single large arch, with the section of wall between the small and large arches pierced by a circular or four-lobed opening. The complexity of this plate tracery increased, reaching a climax in the magnificent windows of Chartres Cathedral. After c. 1220 windows began to be subdivided by mullions, or upright bars, that continued at the head of the window to branch and form the patterns of bar tracery. Elaborate bar tracery soon became one of the most important elements of Gothic architecture and one of its finest achievements, as in the rose windows of the French Rayonnant style. The bar tracery of the parallel English Decorated style formed netlike patterns based on the circle, arch, trefoil, and quatrefoil. By the late 14th century, the Perpendicular style replaced curvilinear tracery with straight mullions extending to the top of the main arch, connected at intervals by horizontal bars.