to obtain a clerestory? Whatever the true process of evolution may have been, it is at least possible that the above explanations are correct and that the chevet vault developed directly from the difficulty of placing windows beneath the ribbed half dome. If such was the case another type of vault would seem to have owed its origin in large part to the lighting problem. TYPES OF CHEVET VAULTS Once introduced, four types of chevet vault were gradually established, not counting the variation which each of them underwent. For convenience these will be called the radiating-ribbed type, the broken-ribbed type, the buttressing-ribbed type, and finally the diagonal or cross-ribbed type. Each will be considered in turn and an effort made to trace their consecutive development. The chronology of these vaults is very difficult to determine. In fact, it is probably safe to assume that the earliest example, if there were not a number of these vaults simultaneously constructed, has disappeared. In any event, it would seem that the vault must have been first used somewhere between 1130 and 1150 as there are several existing examples which date from this period. If these cannot be arranged in any certain order, they may at least, be used to show the form of the early chevets. RADIATING-RIBBED CHEVETS Perhaps the most primitive, in appearance at least, is that above the transept of Tournai cathedral (Fig. 53) in which, as has been noted,[356] the extrados of each rib is built up until it forms a flat sloping upper surface, each cell of the vault proper rising from the ramps thus formed. Next to this vault at Tournai, and as a matter of fact, probably of earlier date though of more developed type are the two chevet vaults of Largny (Aisne) (cir. 1140).[357] and Azy-Bonneil (Aisne),[358]--which are three-celled,--and the one in the lower story of the chapel of the Bishop’s palace at Laon (cir. 1137-1147) (Fig. 62) with five cells. The latter shows their general characteristics. There are no wall ribs and the round-headed windows are only partly raised above the impost of the radiants while there abut against the keystone of the apsidal arch (Plate II-b). It will be noticed also that this arch is greatly thickened to resist the pressure of these ribs, and at Tournai is preceded by a tunnel-vaulted bay to make this resistance even more secure.[359] [Illustration: FIG. 62.--LAON, CHAPEL OF THE BISHOP’S PALACE.]