+DEVELOPMENT OF VAULTING.+ It was in Central France, and mainly along the Loire, that the systematic development of vaulted church architecture began. Naves covered with barrel-vaults appear in a number of large churches built during the eleventh and twelfth centuries, with apsidal and transeptal chapels and aisles carried around the apse, as in St. Etienne, Nevers, +Notre Dame du Port+ at Clermont-Ferrand (Fig. 96), and +St. Paul+ at Issoire. The thrust of these ponderous vaults was clumsily resisted by half-barrel vaults over the side-aisles, transmitting the strain to massive side-walls (Fig. 97), or by high side-aisles with transverse barrel or groined vaults over each bay. In either case the clearstory was suppressed--a fact which mattered little in the sunny southern provinces. In the more cloudy North, in Normandy, Picardy, and the Royal Domain, the nave-vault was raised higher to admit of clearstory windows, and its section was in some cases made like a pointed arch, to diminish its thrust, as at +Autun+. But these eleventh-century vaults nearly all fell in, and had to be reconstructed on new principles. In this work the Clunisians seem to have led the way, as at +Cluny+ (1089) and +Vézelay+ (1100). In the latter church, one of the finest and most interesting French edifices of the twelfth century, a groined vault replaced the barrel-vault, though the oblong plan of the vaulting-bays, due to the nave being wider than the pier-arches, led to somewhat awkward twisted surfaces in the vaulting. But even here the vaults had insufficient lateral buttressing, and began to crack and settle; so that in the great ante-chapel, built thirty years later, the side-aisles were made in two stories, the better to resist the thrust, and the groined vaults themselves were constructed of pointed section. These seem to be the earliest pointed groined vaults in France. It was not till the second half of that century, however (1150-1200), that the flying buttress was combined with such vaults, so as to permit of high clearstories for the better lighting of the nave; and the problem of satisfactorily vaulting an oblong space with a groined vault was not solved until the following century. +ONE-AISLED CHURCHES.+ In the Franco-Byzantine churches already described (p. 164) this difficulty of the oblong vaulting-bay did not occur, owing to the absence of side-aisles and pier-arches. Following this conception of church-planning, a number of interesting parish churches and a few cathedrals were built in various parts of France in which side-recesses or chapels took the place of side-aisles. The partitions separating them served as abutments for the groined or barrel-vaults of the nave. The cathedrals of +Autun+ (1150) and +Langres+ (1160), and in the fourteenth century that of Alby, employed this arrangement, common in many earlier Provençal churches which have disappeared.