The Norman groined aisles are, however, of a different sort, for they either have level crowns or are but slightly domed up in type.[138] The abbey church of Jumièges (Seine-Inférieure) (1040-1067) is among the earliest examples of this construction and is the only Norman church with groined vaults in both the aisles and triforium.[139] La Trinité at Caen[140] and the abbey church of Lessay (Manche)[141] are also Norman churches with groined aisles, in both cases with level crowns. In La Trinité, as in the early churches of Poitou, the bays are not even separated by transverse arches.[142] In Saint Étienne at Caen, and in the choir of the cathedral of Gloucester, the aisles are vaulted in both stories like those of Auvergne, the lower groined, the triforia with half tunnel vaults, but it seems very probable that these latter were added only when vaulting took the place of the wooden roof in the central portions of the church.[143] Curious instances of the persistence of groined vaulting are to be seen in the triforia of such transitional churches as Saint Germer-de-Fly (Oise)[144] and Vézelay, where the remaining portions of the church have ribbed vaults. For this persistence an explanation is later attempted.[145] AISLES WITH SEMI-GROINED VAULTS An unusual form of aisle vault appears at Creully (Calvados)[146] (twelfth century), where the aisles are covered with a half tunnel vault intersected toward the outer wall by lunettes, which thus convert it into a semi-groined vault. Its obvious advantage lies in the combination of inward pressure, which it exerts in support of the nave vaults, with the added window space which it affords without increasing the height of the exterior walls. RIBBED VAULTS The introduction of ribs beneath the diagonal intersections of groined vaulting gradually brought about a revolution in Mediaeval building, and transformed the massiveness of Romanesque construction into the light and graceful architecture of the Gothic era. Much has been written in an effort to discover the origin of the new system. It is not, however, the intention here to add to the number of theories advanced, except in an incidental manner, but rather to classify the various forms of ribbed vaulting as applied to naves, choirs, and aisles of the churches following immediately after those of the Romanesque period which have just been described. As a geographical basis is no longer practical for such a classification, because of the widespread distribution of the new method of construction, a structural basis will be substituted, and the vaults will be divided into two major groups according as they were used over square or rectangular nave bays, and then subdivided according to their minor characteristics. RIBBED VAULTS OVER NAVES WITH SQUARE BAYS