This, as also divers other things, was now instituted and observed amongst God’s people, and afterwards was expressed in a written law, Deu_25:5,6. See also Num_36:6,7 Rth 1:11 Mat_22:24.
Raise up seed to thy brother; beget a child which may have thy brother’s name and inheritance, and may be reputed as his child. So it was with the first child, but the rest were reputed his own.