The Egyptians believed that the soul had three components, the ba, ka, and akh, each of which had different roles after death. The ka remained near or within the body (which is why mummification was required). The ba went to the underworld where it merged with aspects of Osiris, but was allowed to periodically return (which is why Egyptian tombs often contained narrow doors). The akh could temporarily assume different physical forms and wander the world as a ghost of sorts. In the underworld, the ba was subjected to the Judgment of Osiris in the Hall of Double Justice, where the heart of the deceased was weighed against Ma'at, commonly represented as an ostrich feather.