A federal judge blocked an Oklahoma amendment on Monday that would prevent state courts from relying on Sharia law. She issued the restraining order after the director of the Council on American-Islamic Relations in Oklahoma, Muneer Awad, filed a suit claiming the measure was unconstitutional and that it would effectively nullify his Islamic will. * What, exactly, makes a will Islamic?

Incredibly specific inheritance laws. Verses 11, 12, and 176 in the fourth book of the Quran lay out exactly how a Muslim should dispose of his wealth after death. Two-thirds must be distributed as follows, when applicable: One-sixth should go to your father; one-sixth to your mother; half or one-quarter to your husband, depending on whether you’ve had children; and one-quarter or one-eighth to your wife, also depending on whether you’ve had children. As for those hypothetical kids, the fractions vary, but boys get two shares for every one share that girls get.

Sunnis and Shi’ites disagree on how to allocate the remaining one-third. Sunnis can give it to whomever they choose, so long as the recipient isn’t also getting a piece of the initial two-thirds. Shi’ites do not have the same prohibition; they can bequeath all or part of the last third to a favored relative or, if they want to even out the 2-to-1 gender differential mentioned above, bequeath it to their daughters.

Depending on the number of relatives, the calculations that go into determining these fractions can become quite complicated. Back in the 800s, a Persian mathematician named al-Khwarizmi wrote a book called Kitad al-jabr wal-muqabalah that laid out the principles of algebra. One of the problems al-Khwarizmi’s book addressed was the calculation of inheritance under Islam. These days, Muslim lawyers drawing up wills for their clients will often use software programs to determine the various shares.

In a Muslim society in which Sharia law also functions as civil law, it is not necessary to have a will, since Islamic inheritance laws are the default. In places outside of the Islamic world where wills are necessary (like the United States), Muslims sometimes include a preamble declaring their faith in Allah—but this is not essential; the fact of having an Islamic will is itself a declaration of faith. Islamic wills also frequently include specific instructions on funeral and burial rites, as dictated by the Quran. But since Muslim custom calls for burial as soon as possible after death, those questions are often moot by the time the will is read.

As for why these laws were developed in the first place, the Quran states: “[Y]our parents and your children—you know not which of them is more deserving of benefit from you.” That’s been interpreted to mean that in Islam, only God can make distinctions between one relative and another. So it’s safer to follow pre-determined inheritance shares. From a social and economic perspective, the emphasis on distributing more wealth to sons serves the purposes of a patriarchal society, in which men are the breadwinners and sons bear the responsibility of taking care of their aging parents.

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Explainer thanks Shaykh Mohammed Amin of the Daral Qasim institute; Leor Halevi, author of Muhammad’s Grave: Death Rites and the Making of Islamic Society; Hyder A. Naqvi of Ahmad & Naqvi; David Powers of Cornell University; Amina Saeed of the Muslim Bar Association of Chicago; and Ahmed Shaikh.Like Slate on Facebook. Follow us on Twitter.

Correction, Nov. 15, 2010: This article originally used a masculine pronoun to refer to the federal judge who blocked the Oklahoma amendment, Vicki Miles-LaGrange. ( Return to the corrected sentence.)