The simple forms of ribbed vaulting just discussed were the ones most frequently in use during the best Gothic period. But among certain builders, there was a tendency even in the thirteenth century to introduce additional ribs into the vaults, a custom which later gave rise to a vast number of complicated vaulting systems especially in England, Spain and Germany. Even to enumerate these would be almost impossible and a description of each is out of the question, hence only those combinations which were frequently employed, or which gave rise to new types, will be discussed. ORIGIN AND USE OF THE RIDGE RIB Naturally enough the ridge rib was the first to be added to those already constituting the four-part vault (Plate I-j). But the vaults thus formed should be divided into two groups. The first most frequently found in France and already discussed in connection with the churches of Anjou,[222] is that in which the surface of each severy has a curved crown and the rib follows this curve, with the object, probably, both of subdividing the large rectangular bays, of marking with absolute exactness the crown line, and of aiding in rigidly fixing the central keystone, or even in the case of a six-part vault, of giving the same apparent division to the transverse severies as is found in those running longitudinally. Though very similar to this first type, the second, which was developed and most used in England, is different, in that the ridge line is here [Illustration: FIG. 35.--LINCOLN, CATHEDRAL.] perfectly horizontal, and the main purpose of the rib is to mark this horizontal line with absolute exactness and to give, what Bond terms a spine,[223] to the vault skeleton. In the earliest example in England, the transept aisle of Ripon cathedral (cir. 1170),[224] the ribs are so small as to be purely decorative. This leaves the choir of Lincoln cathedral (begun 1192) (Fig. 35) as the first English example of importance in which a true ridge rib appears. It is not yet absolutely horizontal since there is a slight curve to each severy. Its presence would seem to be due to the peculiar form of the vault, in which the ribs enclosing the window cells do not meet at a common point of intersection but at two points somewhat distant from each other along the ridge line where each pair is abutted by a single rib running to the nearest impost on the opposite wall (Plate I-l). This arrangement, which was probably planned to increase the amount of centering in the large transverse panels and thus render their construction easier,[225] gives an extra keystone in each bay and it is quite possible that the ridge rib was introduced in order to unite these intersections and fix them in a straight line. It does not appear in the window cells where it would of course have been at an awkward angle with the outer walls.