The primitive method of vaulting adopted in the central provinces of France in the construction of churches with three aisles rendered such buildings of necessity low and heavy. The main aisle being covered by a barrel vault, supported on either side by a continuous half-barrel vault, the sole means of lighting lay in the windows of the side aisles, so that the nave was always gloomy in the extreme. The Norman architects had avoided this difficulty, first in their native province, and afterwards in England, by vaulting the subordinate aisles only, and by raising the lateral walls of the nave high enough to allow a line of windows to be introduced between the lean-to roofs of the side aisles and the nave roof, the latter being an open timber construction instead of a vault. The lateral gallery in the first story of Norman churches built on the basilican model is merely a development of the ancient tradition.[6] It bears the name of triforium because--or so we are told--each compartment of such an interior gallery between the main piers of the nave was originally divided into three by pillars supporting lintels or by small columns supporting an arcade. [6] See _L'Architecture Romane_, by Ed. Corroyer; Maison Quantin, Paris, 1888, chaps. i. iii. and iv. Towards the close of the eleventh century Norman architects on both sides of the Channel were raising vast churches, the side aisles of which bore above their ribbed vaults galleries after the fashion of the primitive basilicas. These galleries in their turn were covered by open timber roofs like that of the nave. The bays were emphasised in the nave and in the side aisles by transverse arches, or _arcs-doubleaux_, which served as buttresses to those of the main vault. But after the adoption, towards the middle of the twelfth century, of the Angevin method of vaulting for religious buildings, the functions of the lateral walls and of the supporting arches became better defined, for these walls and arches had now to meet the thrusts of the transverse as well as that of the diagonal arches, which, meeting in bundles, as it were, at each pier, gathered their energies at well-marked points. It was thus that the cross walls or _arcs-doubleaux_ of the side aisles were gradually modified till they became detached semi-arches concealed beneath the outer roof of the side aisles. We have traced this modification in the Abbaye aux Dames at Caen.[7] [7] _L'Architecture Romane_, by Ed. Corroyer; Paris, Maison Quantin, 88, chap. xvii. Fig. 24 shows us an English example. It may be followed out in a number of other churches in England, at Pavia in Italy, at Zurich in Switzerland, and at Basle on the Rhine, to name but a few of the churches in which the modification of the vaults was long posterior to the construction of the building itself. [Illustration: 24. DURHAM CATHEDRAL. TRANSVERSE SECTIONS]