like a camel in a gutter

A few decades later marriage was forbidden up to the seventh degree of separation, or “as far as memory could go back.” This made it illegal to marry a descendant of one’s great-great-great-great-great-grandfather! By the end of the eighth century it was incestuous to marry in-laws, the kin of godparents or godchildren, or a relative of someone you had once had sexual intercourse with. It was also forbidden to marry a relative of someone you had previously promised but failed to marry. These prohibitions were so broad that almost any match could be ruled invalid. One historian notes that, at least in theory, the incest rules prohibited young village men from wedding “all the marriageable girls they could possibly know and a great many more besides.”

[…]

On the other hand, because the rules were so routinely ignored at the time of marriage, they could later be used to invalidate an existing marriage. A claim of incest provided an escape clause from the Church’s strictures against divorce. A married couple, or one of the pair, who desired a divorce might suddenly discover that the marriage had been incestuous all along. In the late eleventh century the Count of Anjou was able to repudiate five of his own marriages by dint of diligent genealogical research. In 1152 the divorce of King Louis VII of France and Eleanor of Aquitaine was approved when the couple pointed out that they were related within four or five degrees. The excuse served, even though this had been common knowledge at the time of their wedding and Louis’s next wife, Constance of Castile, was even more closely related to him than Eleanor had been. As one historian notes, the church’s “abhorrence of incest provided a gap in the laws of monogamy through which a king . . . could drive a coach and horses.”

In 1215 the Fourth Lateran Council narrowed the definition of incest to four degrees of separation. The council’s stated aim was to enforce the modified ban more stringently. But popes continued to grant dispensations for political or financial gain. Under the papacy of Boniface IV (1389-1404), marital dispensations were openly available for sale, with a sliding fee scale based on the value of the concession being sought.”]