where the apse is greater than a semicircle--though even this chevet is not of the perfected diagonal-ribbed type, since it has no wall ribs and, moreover, is used over an apse of semicircular instead of polygonal plan like those of the developed Gothic period. An example of the perfected vault may be seen, however, above the apse of Amiens cathedral (Fig. 69). Here there are but five bays of the chevet along the curve of the apse proper, the remaining two being continuations of the choir walls (Plate II-g). The diagonal ribs which determine the position of the keystone are therefore precisely such ribs as those in the remainder of the chevet except that the bay in which they lie is of smaller size than those preceding it and thus forms a gradual transition to the still smaller bays comprising the apse proper. As a result of this arrangement of ribs at Amiens, the keystone of the vault is so placed that it not only renders all the radiants of practically equal length but also makes the crown lines of each window cell so nearly perpendicular to the wall as to give a most symmetrical effect to the entire vault. Such a chevet constitutes the finest method of apse vaulting developed in Gothic architecture and in fact may well be considered the most perfect type conceivable, at least from the point of view of appearance. It loses a little in structural character through the fact that the first ribs do not abut the four eastern radiants at as firm an angle as in the previous chevet type,[380] but the advantage gained in the more symmetrical character of the vaulting severies makes up in large degree for this possible fault. CHEVETS WITH ADDED RIBS [Illustration: FIG. 69.--AMIENS, CATHEDRAL.]