Although usually confined to the side aisle bays, there are a few Romanesque churches in which the builders of the eleventh and twelfth centuries placed groined vaulting over the nave. The scarcity of such examples is due primarily to the difficulty of meeting the severe outward thrusts of a groined vault raised over bays of considerable span and at a point high above the ground. In the side aisles where the vaults were comparatively low, the exterior wall could be thickened by salient buttresses, and the piers strengthened by the weight of the wall above in a manner to offset the thrust, but in the nave the problem was more complicated. The builders had not yet invented the flying buttress. Hence, when they attempted groined vaults at all, they blundered along trusting that the inert mass of their walls and such timid buttresses as could be erected above the nave piers would provide sufficient offset for the thrusts even though these were now concentrated at four main points in each bay. Naturally the vaults frequently gave way and had to be reconstructed. In spite of these difficulties, the advantage of the groined vault in providing a clerestory whose windows might rise as high as the crown of the vault itself led to its occasional use. GROINED VAULTS OVER RECTANGULAR NAVE BAYS The vaults thus employed were of two rather distinct classes, those over rectangular nave bays which were usually but little domed up, and those over square bays which were generally distinctly domed in the Byzantine manner. Of the first type perhaps the best known example is the Burgundian church of La Madeleine at Vézelay (Yonne), (Fig. 17) dedicated [Illustration: FIG. 17.--VÉZELAY, LA MADELEINE.]