+STRUCTURAL DEVELOPMENT: VAULTING.+ By the middle of the twelfth century the use of barrel-vaulting over the nave had been generally abandoned and groined vaulting with its isolated points of support and resistance had taken its place. The timid experiments of the Clunisian architects at Vézelay in the use of the pointed arch and vault-ribs also led, in the second half of the twelfth century, to far-reaching results. The builders of the great +Abbey Church+ of +St. Denis+, near Paris, begun in 1140 by the Abbot Suger, appear to have been the first to develop these tentative devices into a system. In the original choir of this noble church all the arches, alike of the vault-ribs (except the groin-ribs, which were semi-circles) and of the openings, were pointed and the vaults were throughout constructed with cross-ribs, wall-ribs, and groin-ribs. Of this early work only the chapels remain. In other contemporary monuments, as for instance in the cathedral of Sens, the adoption of these devices was only partial and hesitating. [Illustration: FIG. 116--PLAN OF NOTRE DAME, PARIS.] [Illustration: FIG. 117.--INTERIOR OF NOTRE DAME, PARIS.] +NOTRE DAME AT PARIS.+ The next great step in advance was taken in the cathedral of +Notre Dame+[23] at Paris (Figs. 116, 117, 125). This was begun, under Maurice de Sully in 1163, on the site of the twin cathedrals of Ste. Marie and St. Étienne, and the choir was, as usual, the first portion erected. By 1196 the choir, transepts, and one or two bays of the nave were substantially finished. The completeness, harmony, and vigor of conception of this remarkable church contrast strikingly with the makeshifts and hesitancy displayed in many contemporary monuments in other provinces. The difficult vaulting over the radiating bays of the double ambulatory was here treated with great elegance. By doubling the number of supports in the exterior circuit of each aisle (Fig. 116) each trapezoidal bay of the vaulting was divided into three easily managed triangular compartments. Circular shafts were used between the central and side aisles. The side aisles were doubled and those next the centre were built in two stories, providing ample galleries behind a very open triforium. The nave was unusually lofty and covered with six-part vaults of admirable execution. The vault-ribs were vigorously moulded and each made to spring from a distinct vaulting-shaft, of which three rested upon the cap of each of the massive piers below (Fig. 117). The +Cathedral+ of +Bourges+, begun 1190, closely resembled that of Paris in plan. Both were designed to accommodate vast throngs in their exceptionally broad central aisles and double side aisles, but Bourges has no side-aisle galleries, though the inner aisles are much loftier than the outer ones. Though later in date the vaulting of Bourges is inferior to that of Notre Dame, especially in the treatment of the trapezoidal bays of the ambulatory.