Old Persian was the ancient Persian language written in cuneiform on Achaemenid royal monuments.
Old Persian was used by Achaemenid kings in royal inscriptions from the sixth to the fourth centuries BCE. Its specially adapted cuneiform script contains fewer signs and is more alphabetic in structure than the older Mesopotamian cuneiform systems. The best-known example is the trilingual Behistun Inscription of Darius I.
Achaemenid inscriptions often present the same text in Old Persian, Elamite, and Babylonian. These were different languages serving different administrative and cultural audiences. Old Persian was the language of the royal dynasty, but it was not the empire’s only written language or its sole administrative medium.
Old Persian later developed into Middle Persian, the language strongly associated with the Sasanian period. Avestan is different again: it is the language of important Zoroastrian religious texts, not the standard language of Achaemenid royal inscriptions.