In my theoretical description of this form of vault, I supposed the springers which are next to the wall to rise directly from its face (Fig. 316); but in practice it is better that they should rest upon projecting piers; and it is obvious that for these pilasters or columns may be substituted. The crypt under the Church of the Holy Trinity at Caen is a good example of this class of vaulting (Fig. 319). When we apply the transverse rib to this vaulting, we give it at once a strictly architectural character, as every compartment is now distinctly defined (Figs. 320, 321). [Illustration: Fig. 320.--Crypt under the Choir. View looking West. Canterbury Cathedral.] The complete plan of the springer upon a detached pier now takes the cross form, suggesting the substitution of a cluster of four shafts round a square, or of a larger column, with a capital broken into the cross form. Where, however, the weight to be carried was small, as in crypts whose vaulting supported only the floor above, this enlargement of the pier was obviated by making the ribs die out at their springing one into another, and the groin to commence a little higher up; or sometimes by the awkward expedient of making the outer curve of the rib _eccentric_ with the inner one.[38] [Illustration: Fig. 321.--Crypt, Durham Cathedral.] Where we have already clustered pillars carrying a main arcade, the presence of vaulting on either side adds a new member to the pier, both behind and in front; and if, as is usual in churches, the central vault springs from a higher level, the additional shaft on that side runs up _through_, or rather _by_, the capital of the pier till it reaches the higher springing, thus emphasising the division of the bays throughout their entire height.[39] This multiplication, however, of shafts is by no means essential, as the ribs may be brought, by a little management, on to the capital of a single column, which supports the arcades, and on their other side shafts may be carried up upon corbels to receive the higher groining.