But while pleasing in appearance when used in combination with six-part choir vaults, the chevet with buttressing ribs was not so satisfactory in churches with four-part cross-ribbed vaulting of rectangular plan. A reference to the vault of Soissons cathedral (Fig. 67)[375] will illustrate the faults of such a combination. These lie largely in the three-part vaulted bay. In the first place, though its window cells are practically the same width as those in the remainder of the choir, their crown lines run out at an awkward angle,[376] instead of being practically perpendicular to the outer walls as in the remaining bays of the apse and all those of four-part type. Secondly, the great, triangular, transverse severy is much larger than any of the others in the church and is thus unpleasing when contrasted with them, besides being more difficult to construct because of its larger size. It is not surprising to find, therefore, that a fourth form of chevet was developed and used extensively in churches with four-part vaulting. This chevet, which will be termed diagonal-ribbed, is perhaps the most important distinct type developed in Gothic architecture. DIAGONAL-RIBBED CHEVETS [Illustration: FIG. 68.--CHARTRES, CATHEDRAL.] It has already been noted that there were a number of early apses covered with an elementary kind of chevet which was formed by the use of two diagonal ribs over the semicircle of the apse in exactly the same manner as similar ribs were used in rectangular four-part vaulting. Such a vault as this may have been the prototype of the slightly more developed form to be seen in the radiating chapels of the cathedral of Noyon (before 1167)[377] and in the chapel at the end of one aisle of Notre Dame at Étampes (Seine-et-Oise) (cir. 1160). This latter has one extra rib added in what would have been the eastern bay of such a four-part apse vault subdividing it into two window cells and thus producing a four-celled chevet[378] (Plate II-e). It is exactly this principle, applied on a larger scale and with a further subdivision of this outer bay, which may be seen in such chevets as those of Chartres cathedral (Fig. 68) and Saint Étienne at Caen (first quarter of thirteenth century) (Fig. 70).[379] Of these, the one at Chartres has the more primitive character, for all of its seven bays are on the curve of a semicircle and thus none of the choir proper is included beneath the chevet (Plate II-f). As a result of this increased number of bays, the intersection of the two diagonal ribs which form the first two radiants on each side, lies at a point comparatively near the keystone of the apsidal arch. This gives a certain uniformity to the size and character of the bays, but the vault is not yet perfect, for the ribs are still noticeably different in length, and more important than this the crowns of the window cells are at an awkward angle with the exterior wall. These faults are, however, much less marked in Saint Étienne,