The side-aisles are always covered by intersecting vaults, but that of the nave is invariably a simple tunnel-vault, as in the Southern styles, ornamented by occasional transverse ribs, and which in the church at Issoire is slightly pointed. To support this great vault, a semi-vault is carried over the side-aisles—as shown in the section—which forms a massive and perfect abutment to the thrust of the great arch, besides, as before pointed out, rendering the vault independent of a wooden covering, which, though in some instances supplied, was certainly not originally intended. The defect of this arrangement is of course evident, as compared with the Northern styles, inasmuch as a clerestory was impossible, and the only effective light that could be admitted was through the side-aisles. These churches, however, have an approach to a clerestory not found in that at Fontifroide, before quoted, in having a triforium or range of arches opening into the gallery, which gave a lightness of character to the superstructure, and admitted to a certain extent a borrowed light. [Illustration: 595. Elevation of Chevet, Notre Dame du Port, Clermont. (From Chapuy.) No scale.] Externally, the projection of the buttresses is slight, and they are connected by arches, struck from the same centres as the windows, above which three small arches relieve and ornament the upper part of the nave. The central arch of these is pierced with the small window which lights the upper gallery. Above this is a cornice of more elegance and of greater projection than is usually found in churches of this age. The most beautiful and most admired feature of the style is the arrangement of the chapels of the chevet externally.