The word "ērān" is first attested in the inscriptions that accompany the investiture relief of Ardashir I ("r." 224–242) at Naqsh-e Rustam. In this bilingual inscription, the king calls himself "Ardashir, king of kings of the Iranians" (Middle Persian: "ardašīr šāhān šāh ī ērān"; Parthian: "ardašīr šāhān šāh ī aryān"). The Middle Iranian "ērān"/"aryān" are oblique plural forms of gentilic "ēr-" (Middle Persian) and "ary-" (Parthian), which in turn both derive from Old Iranian "*arya-", meaning "'Aryan,' i.e., 'of the Iranians.'" This Old Iranian "*arya-" is attested as an ethnic designator in Achaemenid inscriptions as Old Persian "ariya-", and in Zoroastrianism's Avesta tradition as Avestan "airiia-"/"airya", etc. It is "very likely" that Ardashir I's use of Middle Iranian "ērān"/"aryān" still retained the same meaning as did in Old Iranian, i.e. denoting the people rather than the empire.