The term Iran derives directly from Middle Persian Ērān, first attested in a third-century inscription at Rustam Relief, with the accompanying Parthian inscription using the term Aryān, in reference to the Iranians.[42] The Middle Iranian ērān and aryān are oblique plural forms of gentilic nouns ēr- (Middle Persian) and ary- (Parthian), both deriving from Proto-Iranian arya- (meaning "Aryan", i.e. "of the Iranians"),[42][43] recognized as a derivative of Proto-Indo-European ar-yo-, meaning "one who assembles (skilfully)".[44] In the Iranian languages, the gentilic is attested as a self-identifier, included in ancient inscriptions and the literature of the Avesta,[45][b] and remains also in other Iranian ethnic names Alan (Ossetian: Ир Ir) and Iron (Ирон).[43]