Old Persian is one of two directly attested Old Iranian languages and is the ancestor of Middle Persian. Like other Old Iranian languages, it was known to its native speakers as ariya (Iranian). Old Persian is close to both Avestan and the language of the Rig Veda, the oldest form of the Sanskrit language. All three languages are highly inflected.

Avestan is an umbrella term for two Old Iranian languages, Old Avestan and Younger Avestan. They are known only from their conjoined use as the scriptural language of Zoroastrianism; the Avesta serves as their namesake. Both are early Eastern Iranian languages within the Indo-Iranian language branch of the Indo-European language family. Its immediate ancestor was the Proto-Iranian language, a sister language to the Proto-Indo-Aryan language, with both having developed from the earlier Proto-Indo-Iranian language; as such, Old Avestan is quite close in both grammar and lexicon to Vedic Sanskrit, the oldest preserved Indo-Aryan language.