The medieval canon law of affinity as an impediment to marriage combined a large range of prohibited degrees with a wide power of dispensation. After the Reformation, however, English law, in line with mainstream Protestant opinion, prohibited marriages within the degrees mentioned in Leviticus, with no provision for dispensation. The prohibited degrees were set out in ‘Archbishop Parker's Table’ in the Prayer Book, beginning with the memorable declaration that ‘A man may not marry his grandmother’. In the nineteenth century, however, some of these restrictions came to be challenged. The classic case was that of marriage with a deceased wife's sister, and it was under this title that successive bills were introduced to alter the law.

Until 1857 the law of marriage was administered by the ecclesiastical courts, according to the canon law. However, the civil courts modified and controlled this canon law by means of the writ of prohibition: canon law was now subordinate to common law, and where the two conflicted the civil courts would over-rule the ecclesiastical courts. Marriage with a deceased wife's sister was illegal, and, as with other impediments to marriage, a case could be brought in the ecclesiastical courts to have such a marriage declared void. A case on these grounds could only be brought during the lifetime of both spouses. Nevertheless, the marriage had theoretically been void ab initio, and even after one spouse had died the survivor could still be proceeded against for incest.