The general principle is correct but what in fact happened was that Tamar was married to Judah's eldest son, Er, who died. She then married his second son, Onan, in order that their first son would legally carry on his elder brother's line. However, Onan resented this, and used coitus interruptus as a method of birth control. This was displeasing to God and he died also. Judah asked Tamar to remain in his household rather than return to her parents as a widow until his youngest son, Shelah, was old enough to be married. However, Judah rather got cold feet, fearing for Shelah's life and delayed matters. In the meantime Judah's wife died and knowing that he was feeling lonely and desperate to fulfill her wifely duty to her first two husbands (i.e. To produce two sons to carry on the legal line of each) she pretended to be a shrine prostitute and waited for him to come past . Sure enough he asked for her services and she accepted from him his staff and personal jewellery as a pledge that he would send her a kid from the flock in payment. Shrine prostitutes were always veiled. Maybe it was dark. Despite technically being in his household, he probably had not seen all that much of her. At any rate Judah failed to recognise her. After their liason, she resumed her widows weeds and did not wait for the pledge to be honoured. Three months later it was reported to Judah that Tamar, having obviously played the harlot, was pregnant. Judah, as head of the family, said that she should be executed by burning, which was the punishment for fornication in those days. However, at her trial she produced Judah's jewellery and staff and stated that he was the father of her baby. Judah acknowledged then that she was in the right rather than himself, as he had not given her to Shelah as wife to raise up children for his first two sons' legal descent line. She was carrying twin boys, so in one fell swoop she produced an heir for each of her husbands. She would have continued to bring them up in Judah's household, although the Bible makes it clear he had no further relations with her. King David, and hence Jesus himself, descended through Pharez, the baby who jumped the queue, and he it is who seems to have been given the rights of the firstborn.
