In Islamic architecture, the four-centered arch has been used since the time of the Abbasids, but it was most widely used in Persian culture and the architecture of the heirs of Timur and the Great Mughals. In iwans, in particular, the four-centered arch is used almost exclusively. The portals of the octagonal mausoleum of Kubbat al-Sulaibiya and the palace of Qasr al-Ashik (both located in Samarra, one of the capitals of the Abbasids), built in the 9th century, are considered the first cases of using such an arch.