The Avestan text corpus was composed in ancient Arachosia, Aria, Bactria, and Margiana, corresponding to the entirety of present-day Afghanistan, and parts of Tajikistan, Turkmenistan, and Uzbekistan. The Yaz culture of Bactria-Margiana has been regarded as a likely archaeological reflection of the early "Eastern Iranian" culture described in the Avesta.
Avestan's status as a sacred language has ensured its continuing use for new compositions long after the language had ceased to be a living language. It is closely related to Vedic Sanskrit, the oldest preserved Indo-Aryan language.
"Avestan, which is associated with northeastern Iran, and Old Persian, which belongs to the southwest, together constitute what is called Old Iranian."[f 1] The Old Iranian language group is a branch of the Indo-Iranian language group. Scholars traditionally classify Iranian languages as "eastern" or "western", and within this framework Avestan is classified as eastern. But this distinction is of limited meaning for Avestan, as the linguistic developments that later distinguish Eastern from Western Iranian had not yet occurred. Avestan does not display some typical (South-)Western Iranian innovations already visible in Old Persian, and so in this sense, "eastern" only means "non-western". That is not to say that Avestan does not display any characteristic innovations of its own – e.g., the sibilant pronunciation of the consonant in aša, corresponding to original /rt/ that is preserved in the Old Persian form (arta), as well as Sanskrit (ṛta).
The Avestan language is attested in roughly two forms, known as "Old Avestan" (or "Gathic Avestan") and "Younger Avestan". Younger Avestan did not evolve from Old Avestan; the two differ not only in time, but are also different dialects. Every Avestan text, regardless of whether originally composed in Old or Younger Avestan, underwent several transformations. Karl Hoffmann traced the following stages for Avestan as found in the extant texts. In roughly chronological order:
Many phonetic features cannot be ascribed with certainty to a particular stage since there may be more than one possibility. Every phonetic form that can be ascribed to the Sasanian archetype on the basis of critical assessment of the manuscript evidence must have gone through the stages mentioned above so that "Old Avestan" and "Young Avestan" really mean no more than "Old Avestan and Young Avestan of the Sasanian period."
The script used for writing Avestan developed during the 3rd or 4th century AD. By then the language had been extinct for many centuries, and remained in use only as a liturgical language of the Avesta canon. As is still the case today, the liturgies were memorized by the priesthood and recited by rote.