Deuteronomy 21:17
But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his. But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.
The rule governs how a father's property is to pass on to his sons. The son acknowledged as firstborn receives a double portion of all that the father has, as the right belonging to the firstborn. The younger brothers share alike in the remainder of the estate.