From the details of arch construction little help is to be derived. The double ring of brick voussoirs, the inner horizontal, the outer vertical, is common to Ctesiphon and to Sâmarrâ, as well as to the Syrian ḥîrahs of the intervening age. The system of arch-building over temporary or permanent centerings has been shown by Dr. Reuther to be practised to the present day, but so far as I am aware, arches set back from the jambs, such as those which were built over temporary centerings in the Sasanian palaces and in Ukhaiḍir, are not present in monumental buildings at a later date. There is no recorded example of this construction at Sâmarrâ. Neither do the horse-shoe arches of the central court afford any conclusive evidence as to date. In all probability the horse-shoe arch was used in Mesopotamia long before Ukhaiḍir was built, and it is used to this day. It appears at Tâq-i-Girrâ, a monument of which the date is not determined, though the classical workmanship of its mouldings indicates a period early in the Christian era;[442] it is found in a Hellenistic vault at Chiusi,[443] and it is common in the churches of Syria. To the north of Mesopotamia there is an early example of its use in the basilica at Mayâfârqîn.[444] As for the methods of vaulting employed at Ukhaiḍir they exhibit no features which are not present in the Umayyad buildings on the Syrian side of the desert, but in some respects, for example in the use of the groin and of the fluted dome, they are in advance of Sasanian construction. I have already called attention to the points of similarity between Ukhaiḍir and Kharâneh. They have a certain weight in the chronological problem although they do not afford decisive evidence as to identity of date. With identical requirements details of structure are apt to remain the same over long periods of time. The loophole windows at Abû Hurairah and at Raqqah,[445] in buildings which must be placed in the middle of the twelfth century, differ little, if at all, from those of Ukhaiḍir and Kharâneh. Nor is the coincidence in the latter two monuments of a decorated chamber to the right of the audience room in itself a determining factor. The same scheme may have existed in Mohammadan palaces later in date than Kharâneh, but unfortunately the later palaces have not been preserved or are not yet adequately explored. Possibly the excavations at Sâmarrâ may throw further light on the subject.