As the transept developed in importance aisles were added, sometimes merely along the east walls, but often along the west as well,[303] and even across the ends, especially in churches where such tribunes provided for a continuation of the triforium gallery.[304] In such transepts the side aisles are vaulted just as those belonging to naves of a corresponding period, and therefore require no discussion here. More important are the chapels which open off of the transept, usually from the eastern wall. In general these consist of a semicircular apse either with or without one or more preceding bays. During the Romanesque period such chapels were generally covered with a half dome sometimes preceded by a tunnel vault as in Saint Georges-de-Boscherville, while after the introduction of ribbed vaults, these and the chevet replace the tunnel vaults and half domes in their respective positions. Sometimes the chapels are square, especially in Cistercian churches. They are then covered either with tunnel vaults, as in Kirkstall Abbey, or with ribbed vaults in the Gothic period. Usually all these radiating chapels are but one story in height, but in the cathedral of Laon, two beautiful chapels more than a semicircle in plan and two stories in height appear, one at the east end of both the north and south aisles of the transept (Fig. 54). These chapels are vaulted with seven-part chevets, and form, with the aisles and tribunes preceding them, veritable churches inside of the cathedral. Chapels of similar character, but practically a full circle in plan and vaulted with a double chevet, are also to be seen in the two lower, stories of the transept of Soissons cathedral. They open off of the aisles and galleries through three slender arches, and the view into them from the transept proper affords one of the finest examples of Gothic perspective. CROSSING VAULTS [Illustration: FIG. 54.--LAON, CATHEDRAL, TRANSEPT TRIFORIUM CHAPEL.]