The Avesta consisted of four books or volumes: the Yasna, the book of hymns; the Yashta, the book of prayers; the Visparatu, the book of righteousness; and the Vidaevadata, or the book of laws. The most important of these books was the Yasna which is composed of seventy-two chapters. Of these, seventeen had been sung by Zarathustra himself and were known as the Gathas
