The masses of masonry on either side of the vault are lightened by the tubes which are characteristic of the vaulting system of Ukhaiḍir (section _a-b_, Plate 4, Fig. 1). One of these tubes pierces the wall on either side, partly above the calottes of the recesses. On the east side the opening of this tube can be seen high up in the wall of the corridor 28; on the west side the tube is not visible owing to the interposition of a stair behind the corridor, but there can be no doubt that it exists. Again towards the top of the vault there is another pair of tubes. The western of these two can be seen through a breach in the wall of the stair which leads from room 89 to the second floor; I infer its eastern counterpart. The vault of the great hall is buttressed by the vaults of the chambers of the ground floor and of the first floor which lie at right angles to it. The wings of the three-storied block, of which the great hall forms the centre, are bounded to the north by the two vaulted corridors 5 and 6 (Plate 17, Fig. 1), the western corridor, 5, being 34 metres long, and the eastern, 6, 34·90 metres long. The vaults are constructed in the usual fashion, oversailing the wall and built of thin slabs of stone, laid vertically in concentric, slightly pointed rings. The corridors lead into the palace yard. The door of the west corridor is much ruined. The door of the east corridor is set in a niche surmounted by a shallow calotte, of which the archivolt is slightly pointed. Below the calotte, between it and the arch of the door, is a second small arched niche, connected by the usual funnel with the top of the door arch. The calotte is outlined by a singular decoration composed of a crenellated motive.[20] The crenellated motive is common in the ornament of Ukhaiḍir and elsewhere, but I am not acquainted with any other example of its application to the archivolt.