But he shall acknowledge the firstborn, the son of the hated, by giving him a double portion of all that is found with him; for he is the first of his strength, the right of the firstborn is his.
English: “He shall acknowledge” — this is recognition of the face. And what constitutes recognition of the face? The facial features up to the nose.
On the daf / in context: If the child’s forehead and features as far as the nose have already emerged while the father is still alive, that is sufficient “acknowledgment” for the child to receive the double portion even if the rest of the birth occurs after the father’s death.
Type of Learning
Derash defining the minimum physical recognition required.
What Is Learned
Emergence of the facial features up to the nose while the father is alive is enough to establish the child’s right to the double portion.