“Teacher, Moses gave us a law that if a man dies, leaving a wife without children, his brother should marry the widow and have a child who will carry on the brother’s name.
Then Judah said to Er’s brother Onan, “Go and marry Tamar, as our law requires of the brother of a man who has died. You must produce an heir for your brother.”
Then Boaz told him, “Of course, your purchase of the land from Naomi also requires that you marry Ruth, the Moabite widow. That way she can have children who will carry on her husband’s name and keep the land in the family.”