It has been customary to call the buildings of the eleventh and twelfth centuries, built in the transition of Roman to Gothic art, Romanesque; but the pointed arch was used in both styles, though, as stated above, less frequently in the earlier one; and it should not, therefore, be taken as the distinguishing characteristic of Gothic architecture. The chief points wherein the Romanesque churches, which were the only buildings of importance constructed at that period, differed from the basilicas were in the methods of vaulting and their consequent effects upon the whole structure, the elaboration of the apse, and the system of connected supports employed. The main characteristics of the style were the same in all Western countries, and these being known, it is not difficult to distinguish the slight differences arising from local causes. In the old basilicas the aisles, whether of one or two stories, were lighted by windows in the lateral walls, while the nave borrowed light from them, and also received it directly from a clerestory rising above the roof of the galleries. As we have seen, these buildings were usually covered by wooden roofs, tunnel-vaults or a series of intersecting vaults thrown across the square formed by two of the columns of the nave, equidistant from each other and from corresponding pilasters in the side walls, being only occasionally used in the aisles. The Western architects of the tenth century continued to build their churches in this manner, and we have a splendid example of a timber roof of this kind, as late even as the twelfth century, in Peterborough Cathedral; but at an early period they sought to replace these perishable roofs by stone vaults. They found the construction of the semi-dome of the apse and the vaulting of the side aisles, either by a continuous tunnel-vault, by a series of semicircular vaults perpendicular to the lateral walls, or by intersecting vaults upon a square plan, comparatively easy; but the vaulting of the nave was a much more difficult matter. The circular tunnel-vault would have been the simplest known method of accomplishing this, but the pressure of a circular vault placed over the nave would have tended to push outward the walls upon which it rested, and this pressure being continuous, it was obviously of no avail to place buttresses at any separate point, and to place a great number, side by side, all along the vault, or, in other words, to greatly thicken the supporting wall, was to take up too much valuable ground space.