Deuteronomy 21:17

A double portion ; literally, a mouth of two ; i . e . a portion (so "mouth" is used in 2 Kings 2:9 ; Zechariah 13:8 ) equal to that of two; consequently, the firstborn inherited twice as much as any of the other sons. Amongst all nations and from the earliest times, the right of the eldest son to pre-eminence among his brethren has been recognized; and in legislating for Israel, Moses so far simply sanctioned a usage he found already existing; the assignment, however, of a double share in the inheritance to the eldest son is a new and special provision, mentioned only here. Beginning of his strength (cf. Genesis 49:3 ).