17 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.

Matthew Henry's Commentary on Deuteronomy 21:17

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.