Who you cannot marry: the other causes

A few more things also stop a marriage from being allowed.

The Fourth Cause: Combination (ijtima'). It is prohibited to marry two sisters together in a valid marriage, and it is prohibited to combine any two women whose relationship would bar marriage between them if one were hypothetically a man: a paternal aunt with her niece, for example, or a maternal aunt with her niece. This applies whether the two are married at the same time, or one is married while already married to the other, and whether the relationship is by blood or by fosterage.

Combination also limits how many wives a man may have at once: a free man may have up to four wives, a slave up to two. Marrying more than the limit at once makes the extra marriages invalid, and a man with four wives may not marry a fifth until a divorced wife's iddah has ended.

The Fifth Cause: Ownership. A woman may not marry her own slave, and a master's marriage to his own slave woman is likewise not valid, since the legal effects of marriage, such as dowry and divorce, would serve no purpose where he may already have relations with her without a marriage contract.

The Sixth Cause: Bringing a slave girl into an existing marriage. A woman already married to a free man may not also marry her own slave-girl to him, nor may he marry them both at once.

The Seventh Cause: Polytheism. Marrying a polytheist woman, an idol-worshipper or fire-worshipper, is not permitted unless she becomes Muslim, and marrying an apostate (someone who has left Islam) is likewise not permitted. A Muslim woman may not marry a man of any other religion. A Muslim man may marry a woman of the People of the Book (one who follows a revealed scripture and a prophet's law, such as Jews or Christians), provided she is not herself an idol-worshipper, though it is better avoided where possible.

The Eighth Cause: Divorce before halalah. A man who has divorced his wife three times may not remarry her, unless she first marries another man after her iddah, that marriage is consummated, and it then ends (by his divorce, his death, or her khula), after which, once her new iddah is complete, she becomes lawful for the first husband again.

The Ninth Cause: Being married to, or in the waiting period of, another. It is prohibited to marry a woman who is already married, or who is observing an iddah, whether from divorce or from her husband's death, including a woman pregnant as part of an iddah, who remains prohibited until she delivers.

The Fourth Cause (Ijtimāʿ): combination falls into two categories, combining maḥram women and combining non-maḥram women beyond the permitted number.

Combining maḥram women in a valid nikah is prohibited: marrying two sisters together, and marrying any two women whose relationship would be mutually prohibitive were either hypothetically male, a phuppī (paternal aunt) with her bhatījī (niece), or a khālah (maternal aunt) with her bhanjī (niece), regardless of whether the relation is by nasab or raḍāʿah, and whether ḥaqīqī, ʿallātī or akhyāfī. The prohibition arises whether both marriages are contracted simultaneously or the second follows the first, and contracting during another's ʿiddah is treated as combining for this purpose.

Combining non-maḥram women is limited by the number the Sharīʿah permits: a free man up to four wives, a slave up to two, whether contracted together or across separate marriages. Marrying beyond four at once invalidates the marriages with the excess; marrying additional women after already holding four likewise invalidates the additional contracts; and a man with four wives may not contract a fifth until a divorced wife's ʿiddah is complete.

The Fifth Cause (Milk: Ownership): a woman who owns a slave may not marry him, and a master's nikah with his own slave-woman is void in substance; the aḥkām of nikah such as mahr and divorce would be inoperative, since he may lawfully have relations with a slave-woman he owns without a marriage contract at all.

The Sixth Cause (bringing a slave-girl into a free woman's marriage): a woman married to a free man may not also give her own slave-girl in marriage to him, nor may he contract marriage with both simultaneously.

The Seventh Cause (Shirk: Polytheism): nikah with a polytheist woman (idol- or fire-worshipper) is not valid unless she embraces Islam, and nikah with a murtaddah (a woman who has apostatised from Islam) is likewise invalid. A Muslim woman may not marry a man of any religion other than Islam. A Muslim man may marry a woman of Ahl al-Kitāb (one who follows a revealed scripture (Zabūr, Tawrāh, Injīl) and the sharīʿah of a prophet sent to her, such as Jews and Christians, even where their beliefs have since deviated and their scripture and sharīʿah been abrogated), provided she does not herself practise idol-worship, though it is preferable to avoid such a marriage where possible.

The Eighth Cause (divorce before ḥalālah): a man who has issued three divorces to his wife, whether in one sitting or over time, renders her ḥarām to him permanently (mughallaẓah), and he may not remarry her, until she marries another man after her ʿiddah, that marriage is consummated, and it then ends by his divorce, his death, or her khulʿ; once her ensuing ʿiddah is complete, she again becomes lawful (ḥalāl) to the first husband. This is termed ḥalālah.

The Ninth Cause (manqūḥah or muʿtaddah of another): marrying a woman already married, or observing an ʿiddah, from ṭalāq or from her husband's death, is prohibited, including a woman whose ʿiddah is a pregnancy from that marriage, who remains prohibited until she delivers.