[1] Such altars were an elevated outdoor enclosure, generally strewed with Kusha grass, and having receptacles for the sacrificial fire; it was of various shapes, but usually narrow in the middle.
They were used in various types of Yajna rituals, of which the lengthiest was the agnicayana, lasting twelve days.
In Vedic times, offerings, often including animals, were burnt in the fire, and fully consumed by it.
In the Agnicayana ritual, the mahavedi (great altar) has a length of 24 prakrama in the east, 30 in the west and 36 in the north and south.
[5] The ahavaniya altar has five layers (citi), representing earth, space and the sky.