Deuteronomy 21:16 - Matthew Poole's English Annotations on the Holy Bible

He may not; it is not lawful, because contrary to the rights and law of nature. Before the son, or, before the face of the son, i.e. in his lifetime, as this phrase is understood, Genesis 11:28, Genesis 16:12 25:18. And when this phrase is rendered before another, it signifies only in the presence of another, but never notes the preference of one person to another, which the Hebrews express in another manner. And this may be added to intimate, that if the eldest son were dead, and had left a child, the father was free to give the right of his first-born unto his second son, rather than to the child of the eldest. Or this phrase may be an aggravation of the fact, whereby his father did in a manner spit in his face, and fasten a reproach upon him in his very sight and presence.