15 If a man have two wives, one beloved, and one hated, and they have borne him children, both the beloved and the hated, and the firstborn son be hers that was hated;

Matthew Henry's Commentary on Deuteronomy 21:15

Commentary on Deuteronomy 21:15-17

(Read Deuteronomy 21:15-17)

This law restrains men from disinheriting their eldest sons without just cause. The principle in this case as to children, is still binding to parents; they must give children their right without partiality.