+BUTTRESSING.+ Meanwhile the increasing height of the clearstories and the use of double aisles compelled the bestowal of especial attention upon the buttressing. The nave and choir of Chartres, the choirs of Notre Dame, Bourges, Rouen, and Reims, the chevet and later the choir of St. Denis, afford early examples of the flying-buttress (Fig. 107). These were at first simple and of moderate height. Single half-arches spanned the side aisles; in Notre Dame they crossed the double aisles in a single leap. Later the buttresses were given greater stability by the added weight of lofty pinnacles. An intermediate range of buttresses and pinnacles was built over the intermediate piers where double aisles flanked the nave and choir, thus dividing the single flying arch into two arches. At the same time a careful observation of statical defects in the earlier examples led to the introduction of subordinate arches and of other devices to stiffen and to beautify the whole system. At +Reims+ and +Amiens+ these features received their highest development, though later examples are frequently much more ornate. +INTERIOR DESIGN.+ The progressive change outlined in the last chapter, by which the wall was practically suppressed, the windows correspondingly enlarged, and every part of the structure made loftier and more slender, resulted in the evolution of a system of interior design well represented by the nave of Amiens. The second story or gallery over the side aisle disappeared, but the aisle itself was very high. The triforium was no longer a gallery, but a richly arcaded passage in the thickness of the wall, corresponding to the roofing-space over the aisle, and generally treated like a lower stage of the clearstory. Nearly the whole space above it was occupied in each bay by the vast clearstory window filled with simple but effective geometric tracery over slender mullions. The side aisles were lighted by windows which, like those in the clearstory, occupied nearly the whole available wall-space under the vaulting. The piers and shafts were all clustered and remarkably slender. The whole construction of this vast edifice, which covers nearly eighty thousand square feet, is a marvel of lightness, of scientific combinations, and of fine execution. Its great vault rises to a height of one hundred and forty feet. The nave of St. Denis, though less lofty, resembles it closely in style (Fig. 120). Earlier cathedrals show less of the harmony of proportion, the perfect working out of the relation of all parts of the composition of each bay, so conspicuous in the Amiens type, which was followed in most of the later churches. [Illustration: FIG. 120.--ONE BAY, ABBEY OF ST. DENIS.]