How is the double portion calculated – two times a portion that all the other brothers get or two-thirds of the whole property? The answer is derived from several verses and the gemara explains why all are necessary. Mist of the proofs are from Joseph’s double portion. The gemara then diverges into the details of how Joseph got the firstborn inheritance and other details of the Jacob/Leah/Rachel/Lavan/Esau narrative are brought. A braita describes various things of which the firstborn receives a double portion and the gemara explains each case.