The fact that six-part vaulting declined rapidly in favor toward the beginning of the thirteenth century, and thus before the era of complicated vaults had begun, probably explains the few variants from the standard type. Of these, the simplest consists in the addition of a ridge rib along the longitudinal vault crown. This appears in one bay of the choir of Lincoln cathedral[195] (Fig. 35), where the crown line is horizontal, and in the great transept of the same church where it rises and falls in accordance with the doming up of the central keystone. The small church of Saint Jacques at Reims (Marne) (1183) (Fig. 29) presents a still better example of this irregular ridge rib. The vault of Saint Jacques would seem from its general appearance to be based upon Anjou models and it is not surprising to find its possible prototype in the church of La Trinité at Angers (Fig. 30). The reason for the employment of the extra rib is probably twofold: first, to lessen the size of the transverse panels; and second, to render the arrangement of the ribs and severies more symmetrical. In England, it is quite possible that it served as a cover-joint as well, but in France this would not seem to hold true, at least in La Trinité, where the stone courses are laid with as much care as those in the simple four-part vaults of Angers cathedral (see Fig. 19). [Illustration: FIG. 29.--REIMS, SAINT JACQUES.] La Trinité at Angers (Fig. 30) is also an important variant of the six-part vault because the impost of its intermediate rib is raised to a considerably higher level than that of the principal transverse arches and the intermediate rib itself is highly stilted. This would seem further evidence that the six-part vault was evolved from the four-part vault in an effort to make the arrangement of the windows more symmetrical in a single nave bay corresponding to two bays in the aisles;[196] for if La Trinité with its series of side chapels, two to each nave bay, had been vaulted in the usual Anjou style and the windows left as they now stand on the axis of each chapel arch, their heads would either have been cut by the wall line of a four-part vault or would have appeared awkwardly placed beneath it. The addition of an intermediate transverse arch and the conversion of the vault into sexpartite form restored the symmetry of piers, arches, and windows. In order, however, to obtain as much light as possible and to produce the effect of square nave bays, these intermediate transverse ribs were stilted and their imposts raised. Nor was this stilting confined to Anjou. It appears a number of times elsewhere often in churches where the ridge rib was not employed for example, in the cathedrals of Bremen and Limburg[197] in Germany, and in those of Ribe,[198] and Viborg in Denmark.[199] [Illustration: FIG. 30.--ANGERS, LA TRINITÉ.]