Deuteronomy 27:11-14 -

Having set up the Law and renewed the covenant in Canaan, Israel was to proclaim upon the land the blessing and the curse of the Law, as already commanded (see Deuteronomy 11:29 ). For this purpose six tribes were to station themselves on Mount Gerizim, and six on Mount Ebal, the former to pronounce the blessing, the latter the curse. (On the situation of these two mountains, see at eh. Deuteronomy 11:29 .) The six tribes by whom the blessing was to be pronounced were Simeon, Levi, Judah, Issachar, Joseph, and Benjamin, all descended from the two wives of Jacob—Leah and Rachel. The tribes by whom the curse was to be uttered were those descended from Zilpah, Leah's maid, viz. Gad and Asher; those descended from Bilhah, Rachel's maid, viz. Dan and Naphtali; with Zebulun and Reuben, both descended from Leah. As, in order to ob-rain a division of the tribes into two equal portions, two of the sons of Leah must be assigned to the second half, Zebulun and Reuben were chosen, probably because the former was the youngest of Leah's sons, and the latter had by his sin forfeited his birthright ( Genesis 49:4 ).