2. The heightening of the choir and the introduction of a new clerestory were also carried out shortly after the middle of the fifteenth century. The height of the former vault of the choir is shown by the vault of the crossing (Fig. 836), which it doubtless resembled, and which was not altered at this time. The former position of the vault of the choir is also indicated by portions of the groins, which have only been partly cut away, and which are still visible, springing directly from the caps of the choir piers at the previous low level (see Fig. 830). The ancient vault of the choir, springing, as it did, directly from the caps of the main piers, would only afford room under the wall rib for small clerestory windows. Traces of several similar small windows may still be observed over the main arcade on the south side of the nave, where they have been built up at some period--perhaps during the alterations in 1829; but the rybats and arches are still traceable. The outline of the old roof may also be observed against the east and west walls of the tower (see Fig. 836), the raglet and a stepped string course above it being yet preserved, and being specially apparent on the east side next the choir. The condition of the walls where they have been heightened for the clerestory also indicates that some of the old vaulting has been cut off, and new masonry substituted. The clerestory is carried up so as to admit of good windows, each with arched head, and divided into two lights by a central mullion (see Fig. 830). The traceried heads have drop arches on the inside. The new vaulting is of simple, but elegant, design. The ribs of the vault are moulded, and are provided, besides the main cross ribs, with ridge ribs and intermediate ribs; and the junctions are all ornamented with large and finely-carved bosses, of which some examples are given (Figs. 837 and 838). The curved panels of the vaults are arranged so as to be either parallel or at right angles to the side walls, so that the joints of the stones composing the different vaulting surfaces are all set at right angles to one another, being the plan usually adopted in England, but not in France. The ribs of the vaults spring from the carved caps (Fig. 839) of short and light vaulting shafts, inserted in the side walls and resting on corbels, considerably above the caps of the main piers (see Fig. 830). The insertion of these shafts in the old masonry is plainly visible. [Illustration: FIG. 837.--St. Giles’ Collegiate Church. Bosses in Choir.]