The Yasna liturgy has 72 chapters, traditionally called hads or has. It is one of the central Zoroastrian rituals and contains prayers, invocations, offerings, and recitations addressed to divine beings.
Its 72-part structure has symbolic importance: the 72 threads of the kusti, the sacred cord worn by Zoroastrians, represent the chapters of the Yasna. The chapters are not all the same age or type. Much of the middle section preserves ancient material, including the Gathas, the hymns traditionally associated with Zarathustra.
A common mix-up is confusing the 72 chapters of the complete Yasna with the 17 chapters of the Gathas. The Gathas form the oldest and most revered inner portion of the liturgy, while the Yasna is the larger ritual collection surrounding them. The later chapters include concluding prayers and material connected with the ceremonial offering to water.