Deuteronomy 21:15-17

Firstborn sons

15 A man may have two wives, but he may love only one of them. Each wife may give birth to a son. The firstborn son may be the son of the wife that he does not love. 16 But when the man shares his things with his children, he must promise this: After his death, the firstborn son will receive what he should have. The man may not choose to give the double share of his things to the son of the wife that he loves. 17 He must recognize that his firstborn son is the son of the wife that he does not love. This son was the first son born to that man. So he must receive the double share of his father's things. That is what God's Law says must happen.

21:17double share The firstborn son would receive twice as much as other sons when his father died.