All the rooms and corridors of the palace are vaulted. Some of the finer vaults are built of brick tiles (for example, over the great hall B and over rooms E, F, I, and I´), but as a rule the vaults are constructed with stones set in mortar, the stones being cut into thin slabs so as to resemble bricks as closely as possible. (_Cf._ the Sassanian palace of Firûzâbâd, Dieulafoy, _L’Art Ancien de la Perse_, vol. iv.) All the vaults, whether of brick or stone, are built without centering, and all are set forward slightly from the face of the wall. (The same construction is found at Ctesiphon, see below, Fig. 109.)[83] The groined vault occurs seven times in the corridor C C´ (Fig. 98), and it is also found in room V. (See my article in the _Hellenic Journal_ above cited.) The fluted dome over room A is bracketed across the corners of the rectangular substructure (Fig. 88). In several cases where a barrel vault terminates not against a head wall, but against another section of barrel vault, it is adjusted to the angles of the substructure by means of squinch arches (Fig. 99). A noticeable feature of the vault construction of Ukheiḍir is the presence of masonry tubes running between the parallel barrel vaults (Fig. 100). The structural purpose of these tubes is to diminish the mass of masonry between the barrel vaults. Whenever two barrel vaults lie parallel to one another, a tube will be found between them, and similar tubes exist between the vault of the cloister O O and O´ O´ and the outer wall. (See too Fig. 95, which shows a tube between a barrel vault and a straight wall.) Over the vaults of the rooms of the annex in the eastern part of the court, and also over the vaults of the fourteen parallel chambers outside the enclosing wall to the north, a false roof is laid (Fig. 103). It serves as a protection against the heat of the sun. Under the eastern annex there are some much-ruined subterranean chambers. A staircase at the south-eastern angle of court D leads down into similar cellars (serâdîb).