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)
From the above verses, it is clear that a Muslim must never marry the following:
A great wisdom lies behind these prohibitions on the grounds of consanguinity, affinity, and fosterage. No social cohesion can exist if people do not keep these prohibitions in their minds while contracting marriages.
Temporary prohibitions are those which arise only on account of certain special circumstances in which the parties are placed. If the circumstances change, the prohibition also disappears. They are as follows:
Regarding this last prohibition, the Qur'an expects Muslims to act with the utmost propriety and righteousness. It lays down:
...but do not make a secret contract with them except in honourable terms, nor resolve on the tie of marriage till the term prescribed is fulfilled. (2:235)
This means that a man must not make a specific proposal of marriage to a woman during the time of her 'iddah after the death of her husband or an irrevocable divorce. However, he can send a message saying, for instance, "I wish to find a woman of good character". But if a woman is in the 'iddah of a divorce which is revocable where raja' (return) is possible, a man must not send her even an implied invitation to marry him, because she is still considered as the lawful wife of the first husband. In fact, this restriction is most beneficial because it prevents a man from becoming an instrument of breaking up a family where there are still chances of reconciliation between the wife and husband even though they are moving away from each other.