The semicircular wall at the east end marked a new stage in the development of the Gothic. The upper walls of the clerestory, just beneath the vaults, and the walls of the triforium below them were entirely filled with glass. The arches of the triforium and arcades were topped with pointed arches, stressing the vertical, and admitted a maximum amount of light from different heights and directions, depending upon the time of day. Light also filtered down into the disambulatory from the upper levels.