How was the double portion of the material blessing divided among sons?
Under the material blessing, the eldest son received a double portion. If a father had three sons, the estate was divided into four parts: the oldest received two parts, and each of the other sons received one part. With two sons, it was divided into three parts, the eldest taking two and the other one.
Les explains that the birthright itself did not create the enmity; the blessing did, because the blessing was the material side of the whole picture. The material blessing specified that the eldest son would get a double portion. So if there were three sons, the father would immediately divide the estate into four parts: the oldest would get two parts, and the others each one. In the case at hand, he would have divided it into three parts, and the eldest would get two and the other one. A man like Esau was interested in that and was not about to give up that double portion. That is why he got so uptight when he found out the blessing had already been given to Jacob. It meant Jacob was now in the line of the Messiah, which meant nothing to Esau, but it also meant Jacob would get a double portion of his father Isaac's wealth. These people were wealthy for their time, with flocks, herds, servants, and even gold and silver.