The palace is exactly oriented, the main rooms and entrance facing east. The building materials are undressed stones laid in a thick bed of gypsum mortar. The stones are used exactly in the shape in which they were furnished by nature, a shape which happened to be that of large rounded pebbles. With such materials accurately coursed masonry is not to be expected. The core of the walls is no more than a mass of concrete with stones bedded at haphazard in the strong gypsum mortar. On the outer surface of the wall, particularly in important chambers, the pebbles are, however, coursed with considerable care, but the face of the walls is necessarily very rough and must always have been covered with plaster. The vaults are constructed of the same unfavourable materials. They were built over a centering on which was laid an inner skin of stones and mortar; when this had hardened it was strong enough to bear the mass of concrete which was built round and above it. Construction of this kind would have been impossible but for the excellent qualities of the mortar. I observed that the vaults both in this palace and at Chehâr Qapû had almost invariably a slight outset from the wall (Plate 52, Fig. 2), as is generally the case in Sasanian vault building, whether in brick or in stone. The vaults are round or slightly ovoid, except in the lower corridor, under the margin of the platform (Plate 54, Corridor 103). Here the vaults are very markedly pointed (Plate 51, Fig. 1), but I should attribute this form not to any conscious predilection for the pointed arch--an arch which was, so far as I am aware, unknown to Sasanian architects--but to an accident inherent in the rude construction of an unimportant part of the building. Occasionally brick was used. I saw fragments of brick among the ruins of the palace of Khusrau, and in Chehâr Qapû some brick vaults are still standing. The walls which were intended to support these massive stone roofs were seldom less than 1·30 metres thick, and sometimes considerably thicker. (In Chehâr Qapû, however, they are not infrequently reduced to a thickness of little over a metre.)