Another interesting, though minor feature of chevet vaulting, lies in the form of the masonry panels and the position of the imposts of the radiating ribs. The position of the latter varies considerably, though it corresponds in general with the impost level of the transverse arches in the nave or choir of the church. In the best period this was generally somewhat above the sill line of the clerestory windows, but in some of the early churches like Saint Germer (Fig. 63), Saint Quiriace at Provins (Fig. 31), and the cathedral of Bourges (Fig. 76), it is below this line, while in a number of later churches, among them Saint Urbain at Troyes (Aube) (1262-1329) (Fig. 73), it is as high as that of the arches forming the window heads. This last chevet is also important as showing a tendency to do away with the flat wall forming the lower portion of each panel and starting the outward curve of the masonry directly from the extrados of the ribs. Although this detracts somewhat from the beauty of the vault by making the curve of its cells too abrupt, it does prevent large portions of the windows from being concealed and therefore gives a more uniform effect to the clerestory.[397] Such an arrangement of the window cells is to be found even earlier in the chevet of Bayeux cathedral (early thirteenth century), where the rib rises from the clerestory string-course but is kept close against the wall to the impost of the window arches so that the effect produced is much like that at Saint Urbain. THE SHAPE OF THE CHEVET CELLS [Illustration: FIG. 73.--TROYES, SAINT URBAIN.] Another feature of chevet vaulting which varies greatly throughout its history, is the comparative height of the crown of the wall rib, or line of intersection, and that of the main keystone; in other words, of the doming up of the vault panels. In this, there is a very wide divergence all through the Transitional and Gothic periods. Thus among the early chevets it will be noted that in some the doming is slight though noticeable, as at Saint Germer (Fig. 63), in others it is very pronounced, as at Vézelay (Fig. 66), while in others the crown of the cells actually curves downward toward the central keystone. This is an exceptional type, of which there is an example in Saint Remi at Reims (Fig. 64). Naturally enough, the [Illustration: FIG. 74.--ANGERS, CATHEDRAL.]