 It is necessarily an approximation, since rule over some regions was sometimes divided among different centers of power, and authority in larger polities was often distributed among several dynasties.For example, during the later stages of the Abbasid Caliphate, even the capital city of Baghdad was effectively ruled by other dynasties such as the Buyyids and the Seljuks, while the Ottomans commonly delegated executive authority over outlying provinces to local potentates, such as the Deys of Algiers, the Beys of Tunis, and the Mamluks of Iraq.The study of the earliest periods in Islamic history is made difficult by a lack of sources.