Under the Shari'ah, marriages between men and women standing in a certain relationship to one another are prohibited. These prohibited degrees are either of a permanent nature or a temporary. The permanently prohibited degrees of marriage are laid down in the Holy Qur'an :

And marry not those women whom your fathers married, except what has already happened (of that nature) in the past. Lo! it was ever lewdness and abomination, and an evil way. Forbidden unto you are your mothers and your daughters, and your sisters and your father's sisters and your mother's sisters, and your brother's daughters and your sister's daughters, and your foster-mothers and your foster-sisters, and your mothers-in-law and your step-daughters who are under your mother-in-law and your step-daughters who are under your protection (born) of your women unto whom you have gone into -- but if you have not gone into them, then it is no sin for you (to marry their daughters) -- and the wives of your sons from your own loins, and that you should have two sisters together, except what has already happened (of that nature) in the past. Allah is ever-Forgiving, Merciful. (4:22 - 24)

1. His mother
2. His step-mother (this practice continues in Yoruba land in Nigeria, where in some cases the eldest son inherits the youngest wife of his father)
3. His grandmother (including father's and mother's mothers and all preceding mothers e.g. great grandmothers )
4. His daughter (including granddaughters and beyond )
5. His sister (whether full, consanguine or uterine)
6. His father's sisters (including paternal grandfather's sisters)
7. His mother's sisters (including maternal grandmother's sisters)
8. His brother's daughters
9. His foster mother
10. His foster mother's sister
11. His sister's daughter
12. His foster sister
13. His wife's mother
14. His step-daughter (i.e. a daughter by a former husband of a woman he has married if the marriage has been consummated. However, if such a marriage was not consummated, there is no prohibition)
15. His real son's wife