The original Avesta is traditionally said to have consisted of 21 nasks, or books. This figure refers especially to the Sasanian-period organization described in the Denkard, a later Middle Persian Zoroastrian work that preserves information about the lost collection.
The 21 nasks were arranged in three groups of seven: Gathic, ritual or manthric, and legal. This structure was linked symbolically to the 21-word Ahuna Vairya prayer, whose three lines contain seven words each. The nasks covered theology, ritual, law, mythology, and commentaries on sacred prayers.
The number does not mean that 21 complete books survive today. Most of the Sasanian Avesta was lost, and the surviving Avesta is a much smaller collection assembled from texts that endured in manuscript and ritual traditions. The Vendidad is the only nask generally regarded as surviving substantially intact, while other nasks survive only in fragments or through later descriptions.