An Archaic Dialect Carried by Memory

Gathic Avestan, the dialect of the seventeen Gathas, was already an old and difficult form of the language by the time most of the rest of the Avesta was composed in the more recent Younger Avestan dialect, and by the Sasanian period, centuries later, no community spoke Avestan of any form as an everyday language. Despite this, the text was preserved close to word for word through oral transmission by priests memorizing and chanting it, well before a workable Avestan alphabet was devised, likely in the Sasanian era specifically to fix the pronunciation of a liturgy that was by then understood only imperfectly by many of those reciting it.

Understanding of the text's meaning was carried alongside it in a separate tradition of translation and commentary into Middle Persian, the Zand, itself eventually written down, which meant that sound and sense were transmitted through two related but distinguishable channels rather than a single unified reading practice. This history is part of why Zoroastrian scholarship and Zoroastrian priestly training treat correct chanted transmission as a discipline distinct from textual comprehension; a priest can be a highly skilled and respected reciter of the Gathas while relying on translation and commentary, ancient and modern, to access their meaning, since accurate sound was for many centuries the primary thing being deliberately conserved.