[Illustration: FIG. 98.--A SIX-PART RIBBED VAULT, SHOWING TWO COMPARTMENTS WITH THE FILLINGS COMPLETE. _a, a_, _Transverse ribs_ (_doubleaux_); _b, b_, _Wall-ribs_ (_formerets_); _c, c_, _Groin-ribs_ (_diagonaux_). (All the ribs are semicircles.)] +SIX-PART VAULTING.+ In the Royal Domain great architectural activity does not appear to have begun until the beginning of the Gothic period in the middle of the twelfth century. But in Normandy, and especially at Caen and Mont St. Michel, there were produced, between 1046 and 1120, some remarkable churches, in which a high clearstory was secured in conjunction with a vaulted nave, by the use of “six-part” vaulting (Fig. 98). This was an awkward expedient, by which a square vaulting-bay was divided into six parts by the groins and by a middle transverse rib, necessitating two narrow skew vaults meeting at the centre. This unsatisfactory device was retained for over a century, and was common in early Gothic churches both in France and Great Britain. It made it possible to resist the thrust by high side-aisles, and yet to open windows above these under the cross-vaults. The abbey churches of +St. Etienne+ (the Abbaye aux Hommes) and +Ste. Trinité+ (Abbaye aux Dames), at Caen, built in the time of William the Conqueror, were among the most magnificent churches of their time, both in size and in the excellence and ingenuity of their construction. The great abbey church of +Mont St. Michel+ (much altered in later times) should also be mentioned here. At the same time these and other Norman churches showed a great advance in their internal composition. A well-developed triforium or subordinate gallery was introduced between the pier-arches and clearstory, and all the structural membering of the edifice was better proportioned and more logically expressed than in most contemporary work.