imposts, the exceptionally large size of the transverse arches, and the lowness of those opening into the apse, it would seem as though this aisle had been planned for domed up groined vaulting of the Bourgogne type, already seen at Paray-le-Monial, and that ribbed vaulting came in before the completion of the ambulatory and was therefore substituted. In any event, these straight diagonals and low apsidal arches combined with the heavy transverse arches and the decidedly domed up character of the vaults themselves produce a much more primitive appearance than is to be seen further north in the contemporary vaults of Saint Leu-d’Esserent (Fig. 81). In the latter, the builders have stilted the apsidal and transverse arches, thus greatly reducing the doming of the vaults. They have also provided an impost for the diagonals which are themselves of the broken type, and in fact the form of the vaults is practically perfected except in the matter of the transverse arches. These are still much heavier than the diagonals, a feature which continues to be manifest though in a less marked degree in many of the ambulatory vaults even of the thirteenth century. They correspond in this respect to side aisle vaulting.[434] Only occasionally, as in the splendid inner ambulatory of Le Mans cathedral (1218-1254), were the ribs all made of the same size. This advance combined with its height and general character may perhaps entitle the ambulatory of Le Mans to rank as the finest in Gothic architecture and the high water mark of the trapezoidal four-part broken ribbed vault. TRAPEZOIDAL AMBULATORY VAULTS WITH ADDED RIBS