Deuteronomy 21:10-17

10When you go to war against your enemies and the LORD your God delivers them into your hand and you take them captive, 11if you see a beautiful woman among them, and you desire her and want to take her as your wife, 12then you shall bring her into your house. She must shave her head, trim her nails, 13and put aside the clothing of her captivity.

After she has lived in your house a full month and mourned her father and mother, you may have relations with her and be her husband, and she shall be your wife. 14And if you are not pleased with her, you are to let her go wherever she wishes. But you must not sell her for money or treat her as a slave, since you have dishonored her.

15If a man has two wives, one beloved and the other unloved, and both bear him sons, but the unloved wife has the firstborn son, 16when that man assigns his inheritance to his sons he must not appoint the son of the beloved wife as the firstborn over the son of the unloved wife.

17Instead, he must acknowledge the firstborn, the son of his unloved wife, by giving him a double portion of all that he has. For that son is the firstfruits of his father’s strength; the right of the firstborn belongs to him.