Extispicy began with a question, not an animal. A client, very often a king weighing a military campaign, a royal marriage or an appointment, formulated a precise yes-or-no question and had it addressed formally to the sun god Shamash and the storm god Adad, the two deities credited with governing truthful divination. A sheep was then sacrificed, and the baru priest opened the carcass to examine its internal organs, above all the liver, which Mesopotamian thought treated as the seat of the animal's life and therefore as a surface the gods had, in effect, just written their answer onto. The lungs, coils of intestine and the gall bladder were read as well, but the liver, with its distinctive lobes, gall bladder duct and a small triangular protrusion called the "finger," carried the weight of the reading and gave the discipline its ancient name, barutu, the craft of the baru.
The interpretation was not improvised. Generations of recorded readings had been compiled into omen compendia, above all the massive series conventionally called Baru, which paired specific liver features with specific outcomes in a fixed, citable body of precedent, closer in structure to a legal casebook than to free intuition. Clay models of a sheep's liver, some inscribed with sample omens and some left blank for teaching, survive from sites including Mari and are read by Assyriologists as training aids for apprentice diviners learning to recognise and name each anatomical feature before they were trusted to read a real one.