No existing groin-vault with ribs can, according to Bilson, be dated before the choir aisles of Durham Cathedral, which were begun in 1093.1 This statement by Bilson is supported by the buildings in Caen. Had there been rib-vaults before 1093, the architects of the churches in Caen would certainly have used them, as Caen was the political capital of Normandy. It was from Caen that William set out to conquer England. Here, from 1065 to 1077, William built the nave of St Étienne, at that time with a flat ceiling. The choir, which has not survived, can hardly have looked different from the choir of St Nicolas, which is in the same town, close to St Étienne, and was built only a few years later, from 1083 to 1093. In St Nicolas the choir has a groin-vault and an apse with a half-dome. From this we can conclude that before 1093 there were no rib-vaults in Caen, and probably none anywhere else.
In 1059, William's wife Matilda had founded a nunnery in Caen called L'Abbaye aux Dames, with the church of Ste Trinité attached to it. The original form of this building can be partly reconstructed. The nave had a flat ceiling, and the aisles had groin-vaults with no transverse arches. The original appearance of the east end is not so clear, but the vault of the choir certainly had no ribs, since even the present building has none, though it was built after the death of Matilda (Figure 5). She was buried in the existing choir in 1083, and left legacies of great value to the nunnery.
The Trinité (Plate 3A) is a veritable museum of vaults. The groin-vault in the choir was presumably built after 1083, but before 1093, or, if we choose to take the date of the completion of the aisles at Durham, before 1096. It has a horizontal ridge, which can be seen better from the loft above the vault than from below. The shape of the