replaces the open triforium gallery of Saint Étienne, made it possible to raise the level of the transverse arches of the vaulting to a point considerably above the clerestory string-course. Furthermore, since the wooden outer roof was probably built after the vaults, it was possible to use diagonals whose crowns were higher than those of the transverse arches, and still place them beneath the roof trusses. With such a skeleton of ribs as a basis, the builders proceeded to erect a four-part vault over each nave bay, or, in other words, enclosing two side aisle arches. Because of the higher impost of the vault ribs, the wall intersection of the vault cells easily cleared the window heads.[179] Curiously enough, however, the builders connected the intermediate piers with a transverse arch having a flat wall built upon it to the level of the crown of the longitudinal vault cells (Fig. 27). There would seem to be several explanations of this innovation. In the first place the pier system of La Trinité is regular, not alternate, and a greater symmetry was obtained by having corresponding transverse arches connecting each pair of opposite piers. Moreover such arches had been used before 1114 in the church of Saint Georges at Boscherville, and quite possibly elsewhere as well,[180] beneath a simple wooden roof, thus tying together the lofty clerestory walls. In the second place, such arches had already been introduced at Saint Étienne, though for a different reason, as has been shown, and must have proved of value in keeping the keystone of the diagonals rigidly fixed, besides having become a characteristic of what was perhaps the major church of the school; and in the third place, such an arch with its wall above aided materially both in carrying a portion of the weight of the vault to the alternate piers and in affording permanent centering, which was needed in Normandy even more than in Lombardy because the Norman vault crown was never more than slightly domed up. Once introduced, this pseudo-sexpartite vault was not restricted to La Trinité but was, as has been said, employed in no less than four of the seven square-bayed Norman churches. At Ouistreham (Calvados)[181] (vaulted cir. 1160), the impost was raised as in La Trinité and pointed transverse arches were used, thus increasing the curve of the diagonals and improving the stability of the vault. More interesting still, however, are the two churches of Bernières-sur-Mer,[182] and Saint Gabriel (Calvados)[183] (both vaulted cir. 1150), for in them the builders have clung so tenaciously to the pseudo form in preference to the true that they have actually moved the windows of each bay toward the intermediate pier in order to use this method without raising the imposts. The latter is particularly interesting because of the extreme flatness of its diagonals for which the intermediate transverse arches must certainly have proved an added support.