He made a compromise; he preserved the tribunes but he installed a narrow coursiere or passageway atop the arches of the tribune, which wound around the pillars. He then made the arches of the tribune wider and taller, allowing more light from the windows of the collateral aisles to enter the nave. These modifications were possible thanks to another new technology, the flying buttress, which reaches over the collateral aisles provide to the upper nave walls, allowing them to be thinner and the windows to be larger. The collateral aisles at Rouen are fourteen metres high compared with twenty-eight metres high vault in the nave. The high clerestory windows of the central nave look out over the roofs of the collateral aisles, and bring more light to the interior. Transept The transept is unusually large and brightly lit thanks to the large rose windows on the north and south and the large windows below them in the triforium of each transept. Overhead, the interior of the lantern tower is visible. The walls of the inside of the north and south facades are richly decorated with tracery, composed of pointed stone arches and sculpture in niches and in the small quadrille panels of the south transept. In the northwest corner is a stairway from 1471 which gave access to the cathedral library. It was updated with Neo-Gothic landings in the 18th century. Choir The Choir is the section of the cathedral at the east which was reserved for the clergy, and in the Middle Ages was separated from the nave by an elaborate screen. It was constructed slightly later than the nave, in the middle of the 13th century, and the style is more unified than in the nave. The beginning of the choir is marked by the retable of the main altar, and the throne of the Archbishop. Beyond that to the east are the stalls where the members of the clergy were seated. The elevation of the Choir is different from that of the nave, being more in the High Gothic style of the 13th century, with three levels. The pillars of the arcade are circular, crowned with capitals decorated with stylised foliage and crochets. Above the arcade is the triforium, or enclosed gallery, and above that the high windows, which form a half-circle. The center of the Choir was substantially refurbished before the 1956 re-opening to repair damage