[Illustration: Fig. 382.] In square vaulting, _one_ such additional rib is more usually introduced in each space (Fig. 381). In very oblong vaults _two_, and even _three_, were often introduced in the side spaces, though only one in the middle spaces (Fig. 382). It is clear that this addition necessitates the use of ridge ribs, as, without them, the point at which the intermediate ribs meet at their apex would want abutment. So reasonable, indeed, was this motive, that we often find the ridge rib to have been omitted between the intermediate ribs and the wall ribs, because there its use ceases. One thing which followed the use of these additional ribs was the curious serrated plan of the filling in. The oblique position of these ribs would, if the plan of the filling in remained unaltered, cause the fillet or reveal of the rib nearly to vanish on one side, and to become very wide on the other. This led them to change the plan. On looking at the top surface of vaulting where the ribs are visible, it is at once seen that this was also necessitated by a structural cause, as without it the filling in would not rest well upon the ribs (Fig. 383). [Illustration: Fig. 383.] [Illustration: Fig. 384.--Westminster Abbey. Vaulting west of the crossing.] No better specimen of this form of vaulting can be referred to than that of the presbytery at Ely, built about 1240 to 1250, and the four bays immediately to the west of the crossing in Westminster Abbey, erected by Edward I. about 1280 to 1300 (Fig. 384). The latter is the more perfect, as having level ridges; the former, curiously enough, having ridges to the side cells which rise from the intersection towards the walls. I may mention that it is very common for vaulting with intermediate ribs to have ridges rising rapidly towards the central boss. The use of these additional ribs became, from the latter part of the thirteenth century, rather the rule than the exception. I may mention early specimens of it at Chester, both in the Chapter-house (Fig. 385) and in the Lady Chapel, the latter with raised ridges; but in each the addition being only in the side cells. The Chapter-house at Wells has the intermediate ribs added throughout to those of the more normal examples at Westminster and Salisbury, giving its vaulting a peculiarly full and rich, though rather crowded, effect. Bosses are usually introduced at all points of meeting, adding greatly to the richness of the whole. [Illustration: Fig. 385.--Chapter-house, Chester Cathedral.]