Other methods, however, were employed. Perhaps the chief among these is the five-part vault, in which the triangular severy nearest the outer wall in a four-part vault is subdivided by a half rib running to the main vault crown (Fig. 46). The advantage of such a system lies in the fact that it permits a more pleasing arrangement of windows in the outer wall, especially in bays of rectangular plan, like those in the Certosa at Pavia and Magdeburg cathedral already discussed, where the windows would otherwise fit but awkwardly beneath the broad low wall rib. The same system was also used in aisles with practically square bays, as, for example, in the cathedral of Coutances (Fig. 82), in Saint Urbain at Troyes and in many English churches.[276] Here, too, the explanation is to be found in the window arrangement, especially in the English and Norman Gothic examples, where these windows are of the slender lancet type, which could not be satisfactorily placed beneath the comparatively low wall rib of a square four-part vault. MULTIPLE-RIBBED AISLE VAULTS With the introduction of ridge ribs, tiercerons, and liernes, the side aisles show the same changes as those which took place in the nave. Simple ridge ribs appear, for example, in Lichfield cathedral, liernes at Worcester, while tierceron vaults could be cited in great number. Fan vaults, too, were used in the aisles, and have already been discussed in connection with those of the nave. Reconstructions sometimes produced an unusual vaulting system like that of Beauvais cathedral (cir. 1284), where transverse arches with tracery spandrels were added across each original aisle bay, giving the vault a pseudo-sexpartite character. True six-part vaulting was by its very nature ill-suited for use in the aisles and is very rarely found. There is an example, however, in Magdeburg cathedral.[277] A desire for novelty also seems to have been the cause of unusual vaults, such as those of Bristol cathedral choir aisles,[278] in which low transverse tracery arches separate the bays and carry a system of ribs which subdivide each bay into two rectangular four-part vaults running lengthwise of the aisle. TRIFORIUM VAULTING