Deuteronomy 21:15–17

“If a man has two wives, the one loved and the other unloved, and both the loved and the unloved have borne him children, and if the firstborn son belongs to the unloved, then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the firstborn in preference to the son of the unloved, who is the firstborn, but he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is the firstfruits of his strength. The right of the firstborn is his.

If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love. He must acknowledge the son of his unloved wife as the firstborn by giving him a double share of all he has. That son is the first sign of his father’s strength. The right of the firstborn belongs to him.

“If a man has two wives, one loved and the other neglected, and both the loved and the neglected bear him sons, and if the neglected wife has the firstborn son, when that man gives what he has to his sons as an inheritance, he is not to show favoritism to the son of the loved wife as his firstborn over the firstborn of the neglected wife. He must acknowledge the firstborn, the son of the neglected wife, by giving him two shares of his estate, for he is the firstfruits of his virility; he has the rights of the firstborn.

“Suppose a man has two wives, but he loves one and not the other, and both have given him sons. And suppose the firstborn son is the son of the wife he does not love. When the man divides his inheritance, he may not give the larger inheritance to his younger son, the son of the wife he loves, as if he were the firstborn son. He must recognize the rights of his oldest son, the son of the wife he does not love, by giving him a double portion. He is the first son of his father’s virility, and the rights of the firstborn belong to him.

“If a man has two wives, and the one is loved and the other one is disliked and the one loved and the one that is disliked have borne for him sons, if it happens that the firstborn son belongs to the one that is disliked, nevertheless it will be the case that on the day of bestowing his inheritance upon his sons, he will not be allowed to treat as the firstborn son the son of the beloved wife in preference to the son of the disliked wife, who is the firstborn son. But he shall acknowledge the firstborn son of the disliked wife by giving him a double portion of all that he has, for he is the firstfruit of his vigor; to him is the legal claim of the birthright.

If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.