While these perplexities, however, were under consideration, several others arose, every one of which led to the introduction of features essential to the perfecting both of the style and construction. The first was the desire to elevate the central vault to a higher level, both for the sake of compensating for the loss sustained when it was brought down below the roof, and also to obtain a greater space for the clerestory windows. This involved, again, the difficulty as to abutment, through its raising the springing of the vault above the roof of the aisles. We have seen that, where reduced to a similar difficulty with the _barrel_ vault, the architects of the south of France had at an earlier period resorted to the _pointed_ vault as having less outward thrust: the same expedient was now had recourse to for _groined_ vaulting, the main arches of which were now--towards the middle of the twelfth century--changed from the _semicircle_ to the _pointed arch_. When the elevation of the clerestory above the aisles was but moderate, this was often found sufficient; but the construction was precarious, and in many instances failed, and a more perfect mode of meeting the case was required. What was demanded was the power to elevate the clerestory with the main vault to any reasonable height above the aisle, without endangering the stability of the structure. Here the recollection of an earlier expedient came to the rescue. It will be remembered that the early barrel vaults were buttressed by _half_ barrel vaults over the aisles, thus doing away with the clerestory. A continuous vault demanded a continuous abutment; but, now that the pressure was concentrated into detached planes, it became sufficient that the abutment also should be in those planes; and though the continuous semi-_vault_ would do away with clerestory windows, detached semi-_arches_ would have no such effect. The thought accordingly occurred of erecting the _arc-doubleau_ of the old semi-vault in open air as a buttress to the main vault of the groined church; and hence that much-admired, and, of course, also much-depreciated feature--_the flying buttress_. The pressure being concentrated upon points, it became also necessary to fortify those points by attached buttresses of considerable projection, such as we henceforth find to have become a leading external characteristic of Mediæval structures. The wall, in fact (where the system was carried to its extreme limits), became a mere _curtain_, needed rather for enclosure than for strength, and capable of being pierced with windows to any required extent; a liberty which the contemporaneous development of stained glass caused to be unhesitatingly taken advantage of.