When considered from the point of view of vaulting, the churches of Provence fall into five distinct groups. The first, illustrated by the chapel of Saint Gabriel near Tarascon (Bouches-du-Rhône),[37] is composed of churches with no side aisles. These are covered with tunnel vaults of semicircular or pointed section, with or without transverse arches and carrying directly the tiles of the roof. The supporting walls are frequently strengthened by a series of interior applied arches in one or more orders thickening the wall at the impost of the vault. Outside of this interior buttressing, which has already been seen in Perigord, the churches of this type are of little structural interest. PROVENCE CHURCHES OF THE SECOND TYPE In the remaining groups, side aisles are always present and these have four distinct vaulting systems. In the first, tunnel vaults are employed throughout the edifice. Saint Nazaire[38] (after 1090), the former cathedral of Carcassonne (Aude), though somewhat removed from the center of the school, illustrates this system. Both nave and aisle vaults rise from the same impost level. The vault of the nave is slightly pointed, those of the aisles are semicircular, and both have transverse arches. It is a simple and practical method of construction, since the aisle vaults furnish admirable abutment for that of the nave, and all three are covered by a gable roof of masonry resting directly upon the vault crowns. Its one great fault is the absence of direct light in the nave, a condition which introduces the problem of lighting a tunnel-vaulted church. THE LIGHTING OF TUNNEL-VAULTED CHURCHES This problem was second only to that of constructing the vaults themselves and, furthermore, it had much to do with the forms which these assumed and even with the plan of the church. When there were no side aisles, windows were cut directly through the outer walls, but to introduce a clerestory above an aisle arcade involved a number of structural difficulties. The side aisle vaults no longer aided in supporting that of the nave, and in fact exerted an inward pressure at a point below its impost where such pressure was most difficult to offset. At the same time, the outward thrust of the central tunnel vault was increased in proportion to its elevation from the ground. The simplest method of meeting these difficulties was to increase the thickness of the clerestory walls, or add simple salient buttresses and trust to good construction to offset the increased thrusts. This was the method adopted by most of the Romanesque builders.[39] It was only in the school of Bourgogne, and under its influence, that the problem received a better solution--which will later be discussed at length--and not until the Transitional and Gothic periods that it was completely solved by dispensing entirely with the tunnel vaults.