To DO THE DUTY OF A BROTHER-IN-LAW — one Hebrew verb, built from yabam, 'a husband's brother.' When a man died childless his brother was to marry the widow, and their first son counted as the dead man's, so his name was not blotted out and the widow was not left destitute (38:8). Genesis shows the custom already operating two centuries before Deuteronomy 25:5-10 writes it into law — including the shaming ceremony for a brother who refuses. Everything in Genesis 38 turns on that refusal: Onan takes the marriage and withholds the duty, and Judah then withholds Shelah. The institution drives the whole book of Ruth, where Boaz performs the nearer kinsman's declined obligation. KJV 'perform the duty of an husband's brother unto her'; NWT 'perform brother-in-law marriage with her.' → first discussed at Gen 38:8