There now remain for consideration before closing this chapter, the ribbed vaults of the aisles and triforia of Gothic churches. Very naturally the general development of ribbed vaulting in the aisles closely parallels that in the nave. In by far the larger number of churches, the side aisle bays are square and covered with simple four-part cross-ribbed vaults. As in the case of the nave, those of early date have many clumsy features. Thus in the aisles of Saint Étienne at Beauvais (Fig. 44)--which, fortunately, retain a few bays of their primitive vaults dating from about 1125--the diagonals are heavy (cir. 20-25 cm. thick)[273] and either square with simple bevelled edges or of single torus section. No wall rib is found and the transverse arches, besides being very thick, are of round-headed form, highly stilted to bring them up to approximately the general vault level. The vault itself is slightly domed up at the crown and besides the [Illustration: FIG. 43.--SENLIS, CATHEDRAL, CHAPEL VAULT.] [Illustration: FIG. 44.--BEAUVAIS, SAINT ÉTIENNE.] [Illustration: FIG. 45.--SENS, CATHEDRAL.] primitive characteristics just enumerated, its panels are composed of small stones roughly joined and in very uneven courses, while the ribs themselves are built up of short voussoirs, which are not combined at their springing in the familiar tas-de-charge of more developed Gothic work. The cathedral of Sens presents in its side aisles (Fig. 45), which date from the twelfth century[274] a slightly different system. The transverse arches are still heavy and semicircular but they are not stilted. The diagonals rise from corner corbels--a fact which may prove that the aisles were originally planned for groined vaulting and thus no provision made for the cross-ribs,--and they are also semicircular, thus giving the vault a decidedly domed up character. This makes these vaults at Sens very similar to Lombard work and it would seem as though their builders had the same object of saving centering by the use of ribs as obtained in Lombardy. There is one apparent advance over those at Beauvais in the presence of a wall rib, but this is of too wide a span to fit under its severy, and it would seem to have been designed to mark the wall intersection of groined rather than ribbed vaulting.