Once introduced into English architecture the ridge rib was destined to play a most important part in its development. In the first place, it provided an easy method of assuring an absolutely level and straight ridge line and was thus especially welcome to English builders, who had been trained in the construction of vaults which were never more than slightly and often not at all domed up, and who were, besides, rather inferior masons, and not particularly skillful in making their masonry courses intersect in a perfect manner. In the second place, it furnished admirable abutment for tiercerons or intermediate ribs,[226] which were perhaps suggested by such a vault as Lincoln choir as being valuable additions to the rib skeleton and were thereafter very generally used to provide more permanent centering and to further reduce the size of the vault panels. It is, however, notable that a longitudinal ridge rib appears added to simple four-part vaults without the introduction of tiercerons or transverse ridge ribs at a comparatively early date in Worcester cathedral choir (after 1224),[227] Westminster Abbey choir (1245-1260), and Gloucester cathedral nave (1245), and that it is used in France in a number of churches where there are no tiercerons.[228] In such cases it serves the primary purpose of clearly marking the ridge line, which is especially difficult to adjust in vaults with level crowns. That it was the longitudinal effect thus produced which was desired is evidenced by the fact that except when there were tiercerons in the longitudinal cells, the transverse ridge rib was rarely added to such vaults (Plate I-k). Among the very few examples are the cathedral of Tulle (Corrèze) (twelfth century) and the fifteenth century chapel of the château at Blois, both of them in France.[229] TIERCERON VAULTING The introduction of a ridge rib was only the first step in the development of multiple rib vaulting. It was not long before the builders, especially in England, began to add intermediate ribs or tiercerons between the transverse arches and the diagonals. These may possibly have been inspired by the extra ribs in the choir of Lincoln cathedral (Fig. 35 and Plate I-l), but whatever their origin they became a common feature of later Gothic and gave rise to what may be termed tierceron vaulting. In the transverse vault severies, which in England were really sections of a tunnel vault because of the level crown line, these ribs acted largely as added centering and as decorative features. But when used in the window cells they served another purpose as well for they enabled the builders to convert the ordinary “ploughshare” curve of the vaulting conoid into a series of flat panels which could be constructed with much less difficulty as far as the laying of the masonry courses was concerned.