The term Iran derives directly from Middle Persian Ērān, first attested in a 3rd-century inscription at Rustam Relief, with the accompanying Parthian inscription using the term Aryān, in reference to Iranians. The Middle Iranian ērān and aryān are oblique plural forms of gentilic ēr- (Middle Persian) and ary- (Parthian), both deriving from Proto-Iranian *arya- (meaning "Aryan," i.e., "of the Iranians"), argued to descend from Proto-Indo-European *ar-yo-, meaning "skillful assembler." In Iranian languages, the gentilic is attested as a self-identifier included in ancient inscriptions and the literature of Avesta,[a] and remains also in other Iranian ethnic names such as Alans (Ossetic: Ир – Ir) and Iron (Ossetic: Ирон – Iron).