While its chief effect was upon vaulting, the lighting problems frequently affected the plan of the church as well. When the nave was without direct light, the aisles were almost always narrowed to permit light to enter from windows in their outer wall. Double aisles were practically impossible,[40] unless the inner aisles had triforium galleries supplied with windows.[41] Nor did the problem of lighting enter merely into the construction of simple tunnel vaulted churches. It was involved with that of all kinds of vaulting throughout the entire Romanesque and Gothic periods. Transverse tunnel vaults like those of Tournus, groined vaults like those of Vézelay, the development of the Gothic chevet from the half domed apse, and the systems of ribbed vaulting which are frequently found in the crossings, aisles, and ambulatories of Gothic churches, all are closely related to the lighting problem. PROVENCE CHURCHES OF THE SECOND TYPE CONTINUED Returning to Provence, it will be recalled that Saint Nazaire at Carcassonne was described as a typical example of the second class of churches of this school, entirely tunnel vaulted, with narrow side aisles whose lateral windows afford the only light with which the nave is supplied. There are, however, a few churches, vaulted like Saint Nazaire, in which the builders introduced a clerestory. Among these is the abbey church of Saint Guilhem-du-Désert (Hérault) (rebuilt at the end of the eleventh century).[42] Here the clerestory is of considerable height, the heads of the windows lying beneath the imposts of the tunnel vaults, a fact which renders this church one of the most developed of the school. Yet this development lies merely in the presence of the windows, and not in any structural advances which made their presence possible. It was because of the excellent masonry of the heavy walls and piers, that the Provence builders dared to attempt this innovation. The vaults themselves are no lighter than before and still carry the entire weight of the roof. In fact, the whole system is one of inert stability, analogous to Roman construction, and exhibits little if any advance toward the elasticity and balanced thrusts which were to characterize Gothic architecture. PROVENCE CHURCHES OF THE THIRD TYPE