These are trustworthy documents in which to study the origin of a building and to note its successive transformations, which latter were far more frequent than total reconstructions. For it was not until the beginning of the thirteenth century that great cathedral churches in any considerable numbers were conceived and continuously executed.[9] [9] It is possible, if not easy, to trace the architectural development of the Middle Ages in a good many cathedrals and churches of the twelfth and thirteenth centuries. We have, however, confined ourselves, for the purposes of our present synthesis, to the churches and cathedrals of the royal domain, and more especially of the Ile-de-France, not only because they served as models for the architects of their day, but because they illustrate in a remarkable degree the various transitions we desire to study. [Illustration: 34. CATHEDRAL OF LAON. MAIN FAÇADE] The great abbey churches founded towards the close of the twelfth century in the royal domain, but continued and finished in the early years of the thirteenth, still preserved a more ancient tradition. Laon, which is derived from Noyon and from the south transept of Soissons, consists of a nave with transepts, and of two-storied side aisles vaulted upon intersecting arches, above which, as at Soissons, rise flying buttresses, which meet the thrust of the main vault. This arrangement of the side aisles proves the continuity of the Norman formulæ, just as the method of construction adopted in the main vault demonstrates the persistent influence of the dome.[10] [10] See chap. i., "The Influence of the Cupola on Gothic Architecture." The admirably constructed main vault is square on plan, each square containing two transverse compartments, after the Angevin method as derived from the Aquitainian dome. Here we find indications that, if the builders of the Church of Laon had fully assimilated this method, their minds were nevertheless not altogether at rest as to the functions of the flying buttress. This was, of course, essential to the piers which received the united thrust of both transverse and diagonal arches. But it was far from logical to reinforce the intermediate piers supporting nothing but the auxiliary transverse arches by abutments identical with those of the main piers. The illogicality so striking at Laon is absent from Noyon. There, on the contrary, the architects--of the original construction--had emphasised the functions of the main piers by buttresses of greater projection and solidity than those accorded to the secondary piers. [Illustration: 35. CATHEDRAL OF LAON. THE EAST END] [Illustration: 36. CATHEDRAL OF LAON. SECTION OF THE NAVE]