The texts of the Avesta, also known as the Zend Avesta, — which are all in the Avestan language — were composed over the course of several hundred years. The most important portion, the Gathas, in older (before the works of Johanna Narten 'Gathic') Avestan, are the hymns thought to have been composed by Zoroaster himself. The liturgical texts of the Yasna, which includes the Gathas, is in Older Avestan, with short, later additions in Younger Avestan. The Yasnas' oldest portions may be older than the Gathas, later adapted to more closely follow the doctrine of Zoroaster. The various Yashts are in Younger Avestan and thought to date to the Achaemenid era (559–330 BC).^[citation needed^] The Visprad and Vendidad, which are also in Younger Avestan, were probably composed even later but this is not certain. The Vendidad were written primarily for ritual purification. The Visparat, another group of writings in the 'Avesta', were believed to be made for the honor and prayer of the celestial lords.[1]