The Iranian languages or Iranic languages form a branch of the Indo-Iranian languages, which in turn are a branch of the Indo-European language family. The speakers of Iranian languages are known as Iranian peoples. Historical Iranian languages are grouped in three stages: Old Iranian (until 400 BCE), Middle Iranian (400 BCE – 900 CE), and New Iranian (since 900 CE). Of the Old Iranian languages, the better understood and recorded ones are Old Persian (a language of Achaemenid Iran) and Avestan (the language of the Avesta). Middle Iranian languages included Middle Persian (a language of Sassanid Iran), Parthian, and Bactrian.
What stage of Iranian language was in use until 400 BCE and included Parthian and Bactrian?