An OT custom of dividing an inheritance into portions – the first-born son receiving a double portion of what the other sons received (Deut 21:17). The birthright could be sold or given away if the oldest son chose, however the leadership role would be forfeited. Esau had little regard for the spiritual blessing that the birthright would bring him and sold it to Jacob for a single meal (Gen 25:29-35).
The father usually waited until late in his life before formally blessing the holder of the birthright. The blessing (distinct from, yet complementary to) made the birthright irrevocable and binding. Upon the death of the father, the son with the birthright became the family leader.