The fire in which Vedic sacrifices are performed, should be generated by attrition. The two pieces of wood used for this purpose are called ‘araṇis.’ The lower piece, termed ‘adharāraṇi,’ is rectangular in shape and has an indentation called ‘devayoni,’ the origin of the god of fire. The upper piece called ‘uttarāraṇi’ is in the form of a drill, which is inserted into the indentation of the adharāraṇi. Fire is generated by vigorous churning, to the chanting of appropriate ṛks (hymns from the Ṛgveda).
There are several rules governing the preparation of these araṇis. The upper araṇi should be made from the wood of the śamī tree (Prosopis specigera) which is hard; and the lower one should be of the aśvattha (Ficus religiosa) which is softer, the size being 16 aṅgulas long, 12 aṅgulas wide and 4 aṅgulas in height.
The lower araṇi is sometimes figuratively called the ‘mother,’ the upper araṇi the father and agni the fire, as the offspring.