The solution of the problem of covering a trapezoidal bay with ribbed vaults lay in the employment of the broken rib, or in other words, in the selection of a point of intersection from which four half arches were extended to the supporting piers (Plate III-c). This system, which was very possibly first employed at Saint Denis (1140-1144),[429] became the standard throughout the best Gothic period wherever trapezoidal bays were used, though there was a certain amount of variance in the position of the keystone. At Saint Denis, and in the great majority of the best Gothic churches it lies practically on the line of a curve through the crowns of the apsidal arches and concentric with that of the apse,[430] but in some instances, notably at Sens cathedral[431] and in the ambulatory of Canterbury[432] which was directly influenced by the first-named church, the point of intersection was moved outward to a point where the line from this crown to the transverse arch is practically perpendicular to the latter. The result is an equalizing in length of the four half ribs, but this is accomplished only at a considerable sacrifice in appearance.[433] METHOD OF CONSTRUCTION IN AMBULATORY VAULTS The actual construction of ambulatory vaulting followed much the same course as that of vaults in the remainder of the church and especially those in the side aisles. Thus in the cathedral of Langres (Fig. 80), which dates from the close of the twelfth century and is somewhat south of the center of architectural development in the Transitional period, the ambulatory presents a number of rudimentary characteristics. In fact, judging from the awkward manner in which the diagonals rise from their [Illustration: FIG. 81.--SAINT LEU-D’ESSERENT, ABBEY CHURCH.]