Deuteronomy 21:15-17

Inheritance Rights of the Firstborn

15If a man has two wives, athe 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,
Or hated; also verses 16, 17
16then on the day when che 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, 17but he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is dthe firstfruits of his strength. eThe right of the firstborn is his.

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