Notre Dame de Paris was begun towards the close of the twelfth century, and finished, save for the chapels, in the first half of the thirteenth. As at Laon, the Norman tradition is observed in the arrangement of the upper galleries of the side aisles, while the influence of the dome is again to be traced in the sex-partite groining. The same illogical system of abutments obtains as at Laon. This vast building, consisting of a nave and double side aisles of equal height sweeping round the semi-circular choir, seems to be one of the first five-aisled cathedrals; its grandiose arrangement, the boldness of its combinations, and the perfection of its detail mark the considerable progress made by the architects of the Ile-de-France. [Illustration: 37. NOTRE DAME DE PARIS. PLAN] The method of construction here adopted has a peculiar significance. The upper internal galleries, vaulted on diagonal arches, and raised considerably above the level of the second side aisle, the boldness of the flying buttress, which at one span embraces the two side aisles and forms the abutments of the main vault--alike prove that the architects of Notre Dame de Paris had adopted the newly discovered systems even to excess, and were applying them with unparalleled skill and ingenuity. [Illustration: 38. NOTRE DAME DE PARIS. SECTION OF THE NAVE] The Norman tradition which had obtained in the Ile-de-France passed away in the first years of the thirteenth century. At Châlons-sur-Marne the nave is flanked by two-storied side aisles. But the upper gallery, vaulted and greatly reduced in size, shows that the conventional arrangement was fast dying out. The influence of the dome was longer lived, as is shown in the construction of vaults at this period. We may still trace it at Langres in the domed form of the vaults, which, in spite of their rectangular plan, seem to be a reduced copy of the Angevin naves. [Illustration: 39. NOTRE DAME DE PARIS. FLYING BUTTRESSES AND SOUTH TOWER] The naves of Sens and of Bourges are also vaulted in square compartments. The thrust of the vaults is carried by the diagonal arches to each alternate pier, the intermediate one receiving only the auxiliary transverse arch already fully described. Yet here again the exterior flying buttresses are all of equal solidity in spite of the varying strain. This arrangement, prudent if illogical, shows once more with what distrust architects had adopted that system of exterior abutment, the characteristic of which is a detached arch exposed to all the vicissitudes of weather, and yet responsible for the stability of the whole edifice.