Middle Persian sources, chiefly the ninth-century Denkard, describe a Sasanian great Avesta in twenty-one nasks or volumes, the number chosen to match the twenty-one words of the Ahuna Vairya prayer and grouped in three sets of seven. The Denkard summarises the contents of each. Set against what actually survives, those summaries make the scale of loss plain: only about a quarter of the material came through, and several nasks are known only as titles and precis.
The surviving Avesta is organised by liturgical use rather than by subject. The Yasna, seventy-two chapters, is the text of the central act of Zoroastrian worship, recited while the ritual is performed. The Visperad is a supplementary set of invocations interpolated into the Yasna at seasonal festivals. The Vendidad, more properly Videvdad, "the law against the demons", is twenty-two chapters of purity regulation, dealing with pollution, corpses, dogs, medicine and penalties, and it is the most controversial part of the corpus, some modern Zoroastrians treat it as a priestly accretion at odds with the ethical religion of the Gathas rather than as scripture on the same footing. The Yashts are twenty-one hymns, each addressed to a particular divinity or divine concept, and they preserve a great deal of pre-Zoroastrian Iranian myth and heroic legend. The Khordeh Avesta, the "little Avesta", is a compilation of the short prayers, the five daily invocations and the passages a lay Zoroastrian actually uses, assembled in the Sasanian period and still the ordinary devotional book.