Genesis 38:8 - Matthew Poole's English Annotations on the Holy Bible

This, as also divers other things, was now instituted and observed amongst God's people, and afterwards was expressed in a written law, Deuteronomy 25:5,6. See also Numbers 36:6,7 Rth 1:11 Matthew 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.