the advantages accruing to the eldest son. These were not definitely fixed in patriarchal times. Great respect was paid to him in the household, and, as the family widened into a tribe, this grew into a sustained authority, undefined save by custom, in all matters of common interest. Thus the "princes" of the congregation had probably rights of primogeniture. (Numbers 7:2; 21:18; 25:14) (Gradually the rights of the eldest son came to be more definite: (1) The functions of the priesthood in the family with the paternal blessing. (2) A "double portion" of the paternal property was allotted by the Mosaic law. ( 21:16-17) (3) The eldest son succeeded to the official authority of the father. The first-born of the king was his successor by law. (2 Chronicles 21:3) In all these Jesus was the first-born of the father.
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Smith, William, Dr. "Entry for 'Birthright'". "Smith's Bible Dictionary".