The arches over the doorways are usually of an ovoid shape, sometimes slightly pointed. When the door-jambs take the form of engaged columns, the capitals of the columns, roughly blocked out in masonry, carry an arch slightly narrower in width than the opening of the doorway beneath it. But when the door-jambs are formed merely by the straight section of the wall, the span of the arch is wider than the opening of the doorway (Fig. 102 illustrates both types). This set-back of the arch was doubtless employed in order to facilitate the placing of centering beams. Three wide doorways with round arches, b b´ and c, lead from the main block of the palace building into the surrounding court. The arches are usually characterized by double rings of voussoirs (_cf._ Ctesiphon and other buildings of the Sassanian and early Mohammadan period), the inner ring laid so as to show the broad face of the stones or tiles, while the narrow end shows in the outer ring. (See the arch in Fig. 102.) The arch construction in the eastern annex is, however, much rougher in style. The outer ring of voussoirs is omitted there, nor is it invariable in other parts of the palace.
The niche plays a large part in the decoration of Ukheiḍir. A row of narrow niches runs along the top of the outer face of the northern enclosing wall, but very little of it is now left (Fig. 87). The southern face of the three-storeyed block bears an elaborate niche decoration (Fig. 91). Here the lowest row of niches forms part of the series already mentioned which runs round court D. Above these, on the second storey, are remains of another row of arched niches, each of which contains three small niches. So far as I know, this feature of a large niche enclosing groups of smaller niches has not yet been observed in Sassanian architecture. It is found, however, in a certain well-known type of early Christian church (see, for instance, Ala Klisse, published by me in the _Thousand and One Churches_, p. 403). On the third storey of the palace the face of the wall has been left blank, but above the windows there are still traces of a third order of small niches. Pairs of niches flanked by engaged columns are to be seen in room G´. They are set high up in the wall between the transverse arches. On these transverse arches there is a plaster decoration, the same in character as that which occurs in the semi-domes at the ends of the vault in Court S (Fig. 101). The motives there used are the flute (in the squinch arch and in the conical segment of the semi-dome above it), and a pattern which resembles a tiny battlemented motive. Upon the transverse arches the battlemented motive is doubled so as to form diamond-shaped patterns. In the centre of each of these diamonds, and in the centre of the tiny arched niches at the bottom of the vault, and also between those niches, there are small funnel-shaped motives formed of concentric rings. Between the transverse arches there is a boldly worked ribbing. The arch round the eastern of the two doors that leads into corridor Q´ is surrounded by cusps (Fig. 94). (_Cf._ Ctesiphon, Dieulafoy, _op. cit._, vol. v. plate 6.) A blind arcade, borne by pilasters, is to be seen in courts M M´ and N N´. In the antechamber U there are shallow niches on either side of the doors.