An illustration of this mode of construction has been attempted in the accompanying drawing, which does not represent any special building, but in which the chief characteristics of the style at this juncture have been introduced. The distance across the nave being usually greater than that between the columns dividing it from the aisles, the rectangular compartments of the vault were consequently no longer square, but oblong, so that while the arches crossing the nave at right angles were still semicircular those between the pillars were pointed. The transition from this, in the thirteenth century, to the definite adoption of the pointed vault was consequently but a step. We see, thus, that a continual progress was made in vaulting throughout the style, and the principle of concentrating weight upon isolated points was evolved in order to vault the nave and at the same time give direct light to it. In effecting this result, however, the original aim had been lost sight of—namely, that of avoiding the use of wooden roofs; for when the Romanesque architects abandoned tunnel-vaulting they had to surmount their more complicated intersecting vaults by wooden roofs, the perishable nature of which caused the ruin of many of the finest buildings. Nor was the external appearance of these roofs any improvement upon those of St. Etienne and St. Sernin, for it is a question whether any more monumental roof has been conceived than that which is formed by the natural outside surface of stone vaults. In the old basilicas, columns taken from or modelled upon those of the temples and palaces of Rome had sufficed to support the light brick wall, carried upon an architrave or arches, which enclosed the nave. When the Western architects resumed the building of churches, after an interval of war and trouble which had proved fatal to architectural progress, brick was little used and the formation of light masonry and good mortar were lost arts. The slender Classic column was consequently insufficient to carry the load of a heavy stone wall and had, necessarily, to be replaced by a more solid pier. These piers assumed various forms in the tentative efforts made to construct them of the dimensions calculated to occupy the least amount of floor space; some were square, others circular or formed of a number of small columns grouped together, but for a long time no very satisfactory shape was found which avoided a clumsy adjustment of the superstructure.