The Yasna is the central liturgical text of Zoroastrianism, seventy two chapters called haitis that a priest, the zaotar, recites in full during the ceremony that shares its name. That ceremony consecrates and offers haoma, a pressed sacred plant, alongside bread and other elements, and the chapters of the Yasna supply the invocations, praises and instructions that carry the rite from its opening to its close.
Embedded within this liturgical frame are the Gathas, seventeen hymns spanning chapters 28 to 34, 43 to 51 and 53, composed in an older form of the language than the rest of the text and traditionally attributed to Zoroaster himself. Their placement inside the Yasna, rather than as a separate book, means the oldest and most personal material in the Zoroastrian corpus reaches a worshipper as the centerpiece of an ongoing rite rather than as scripture read apart from it. Nearby sit the Yasna Haptanghaiti, seven prose chapters close in age to the Gathas, forming a second early layer between the hymns and the surrounding liturgy.
Later chapters address Ahura Mazda and the Amesha Spentas, and closing sections address fire and water directly, both treated as sacred in Zoroastrian practice. Because the Yasna is defined by performance as much as content, its chapters make fullest sense read alongside an account of the ceremony they belong to rather than as free standing prose.