Shulchan Aruch/Even ha-Ezer/15
1. These are the forbidden unions that stem from ervah, those from the Torah and those from the rabbis. Those that are from the Torah, kiddushin doesn't take effect. Those that are from the rabbis, kiddushin does take effect and you must deliver a get. And likewise a betrothal with a doubtful ervah also needs a get.
2. A man's mother is forbidden to him according to the Torah, but his grandmother is only forbidden to him from rabbinic law and this law doesn't stop even through the great great great grandmother.
3. The mother of his father's mother is forbidden to him through rabbinic law, her alone.
4. The mother of his father is forbidden to him through rabbinic law and this law doesn't stop even through the great great great grandmother.
5. The mother of his father's father is forbidden to him through rabbinic law, her alone. His father's wife is forbidden to him through the Torah whether or not she's his wife through nisuin (ie fully married) or through erusin (engaged). Whether or not his father is living or whether he's dead or if he divorced her. (Isserles: But if she is his handmaiden she is permitted to him). But if the father took her in force the woman is permitted to him.
6. His paternal grandfather's wife is forbidden to him according to rabbinic law and this doesn't cease because the wife of Jacob our Father is forbidden for every one of us.
