Šahrestānīhā ī Ērānšahr

'The Provincial Capitals of Iran') is a surviving Middle Persian text on geography, which was completed in the late eighth or early ninth centuries AD.

The text itself has indication that it was also redacted at the time of Khosrow II (r. 590–628) in 7th century as it mentions several places in Africa and Persian Gulf conquered by the Sasanians.

[5] Ardashir I, who was the first king of the Sasanian Empire, had used the older word ērān (Parthian aryān) as part of his titles and in accordance with its etymology.

According to the inscription, after death of Shapur's father and his accession, the Roman emperor Gordianus III “marched on Assyria, against Ērānšahr and against us”.

This shorter term "Eran" appears in the names of the towns build by Shapur I and his successors as well as in the titles of several high-ranking administrative officials and military commanders.

[6] According to the book and as an ancient Iranian tradition, Ērānšahr is divided into four "mythologically and mentally"[8] defined regions or sides called kusts.

These parts/regions/sides of the state during and after Khosrow I, on the pattern of the four cardinal points, are (1) Xwarāsān “northeast”; (2) Xwarwarān “southwest”; (3) Nēmrōz “southeast”; and (4) Ādurbādagān “northwest”.