In their vaulting structures, Umayyad period buildings show a mixture of ancient Roman and Persian architectural traditions. Diaphragm arches with lintelled ceilings made of wood or stone beams, or, alternatively, with barrel vaults, were known in the Levant since the classical and Nabatean period. They were mainly used to cover houses and cisterns. The architectural form of covering diaphragm arches with barrel vaults, however, was likely newly introduced from Iranian architecture, as similar vaulting was not known in Bilad al-Sham before the arrival of the Umayyads. However, this form was well known in Iran from early Parthian times, as exemplified in the Parthian buildings of Aššur. The earliest known example for barrel vaults resting on diaphragm arches from Umayyad architecture is known from Qasr Harane in Syria. During the early period, the diaphragm arches are built from coarsely cut limestone slabs, without using supporting falsework, which were connected by gypsum mortar. Later-period vaults were erected using pre-formed lateral ribs modelled from gypsum, which served as a temporal formwork to guide and center the vault. These ribs, which were left in the structure afterwards, do not carry any load. The ribs were cast in advance on strips of cloth, the impression of which can still be seen in the ribs today. Similar structures are known from Sasanian architecture, for example from the palace of Firuzabad. Umayyad-period vaults of this type were found in Amman Citadel and in Qasr Amra.