Ибн Хордадбех
The accounts of Qudama ibn Ja'far and Ibn Khordadbeh describe the general administrative setup of the 9th-century Sawad. It was divided into 12 districts ("astān"; there were only 10 at Ibn Khordadbeh's time), each of which consisted of several sub-districts ("tassūj"). There were 60 tassujs in total (48 at Ibn Khordadbeh's time). Many of these divisions bear Sasanian names, indicating that they had originally been established before the Islamic conquest of Iraq.