logical continuation of such a vault. But the builders do not seem to have realized immediately the aesthetic advantage in so planning their churches that such chevet vaults should come next to a six-part vault. At Sens (Fig. 28), however, the perfected use of this new chevet is shown for it is placed directly beyond a six-part bay and its two buttressing ribs are the counterparts of the two diagonals of the sexpartite vault. Once the advantage of such an arrangement was grasped, the churches were in many cases planned to provide for an even number of six-part bays in the choir followed by a chevet which carries the same system into the apse of the church. Thus in the cathedrals of Paris and Bourges, and probably originally in that of Soissons, as well as in other churches with six-part vaulting, this chevet became the standard form of eastern termination and the bay preceding the apse was made sexpartite so that the completed church would be uniform throughout.[372] Moreover the apsidal bays of the later chevets, as for example that at Soissons (Fig. 67) were frequently so planned that the radiants from the piers next beyond the ribs forming the transverse arch containing the keystone, were exact extensions of the buttressing ribs. In other words, except for the subdivision of the eastern bay into three window cells, the chevet corresponded to a true six-part vault inscribed in the space formed by the last bay of the choir and the polygonal-sided apse. Notwithstanding the fact that the buttressing-ribbed chevet was primarily suited to churches with six-part vaulting, it was by no means confined to these for it is found in a large number which were from the beginning planned for four-part vaults. Among these is the cathedral of Rouen, in which the chevet is of distinctly six-part type with a full-sized choir bay included beneath the vault,[373] and the cathedral of Reims in which all the bays of the chevet are of practically the same size, as in the early churches which gave rise to this form of apse vault. Reims is thus an example of the perseverance of the design of a seven-sided chevet including one bay with parallel walls and yet of the same size as those forming the curve.[374]