GEN-MEDIEVAL/soc.genealogy.medieval

[Originally posted to soc.genealogy.medieval on 2 Dec. 1997, it first appeared in its final form on 23 Jun 1998]
 
A short exposition of the basics on consanguinity is found in Constance B. Bouchard's article, "Consanguinity and Noble Marriages in the Tenth and Eleventh Centuries," Speculum 56 (1981), 268-87, especially at 269-71. To quote:

"Roman civil law [which was the code adopted by the early Church] had forbidden marriages within 'four degrees' and had computed degrees by counting from one prospective spouse up to the common ancestor and then down to the other partner.... Marriages of first cousins, those between people related within four degrees, were forbidden..."

to illustrate:
2
__|__
|     |
1     3
|__   |
|  |  |
ego 2  4 

"But in the first half of the ninth century, both the number of forbidden degrees was increased--from four to seven--and the method of calculating degrees was changed. Now, rather than counting up from one spouse to the common ancestor and down to the other, one computed degrees by counting generations back *only* to the common ancestor..."

to illustrate:                x
                __|__
                |     |
                x     7
              __|__   |
             |     |  |
             x     6  7
           __|__   |  |
          |     |  |  |
          x     5  6  7
        __|__   |  |  |
       |     |  |  |  |
       x     4  5  6  7
     __|__   |  |  |  |
    |     |  |  |  |  |
    x     3  4  5  6  7
  __|__   |  |  |  |  |
 |     |  |  |  |  |  |
 x     2  3  4  5  6  7
 |__   |  |  |  |  |  |
 |  |  |  |  |  |  |  |
ego 1  2  3  4  5  6  7

[these two tables adapted from diagrams in Bouchard's article]
This new method of counting remained in force through the Middle Ages, though in the Fourth Lateran Council of 1215 the number of degrees within which marriage was prohibited was again reduced from seven to four:

        x
        __|__
       |     |
       x     4
     __|__   |
    |     |  |
    x     3  4
  __|__   |  |
 |     |  |  |
 x     2  3  4
 |__   |  |  |
 |  |  |  |  |
ego 1  2  3  4

Note that in cases where two individuals descended in an unequal number of generations from a common ancestor, the more distant descent governed the degree of consanguinity:

        x
        __|__
       |     |
       x     4
     __|__   |
    |     |  |
    x     3  4
  __|__   |  |
 |     |  |  |
 x     2  3  4
 |__   |  |  |
 |  |  |  |  |
ego 1  2  3  4
 |  |  |  |  |
 |  |  |  |  |
 1  2  3  4  5
 |  |  |  |  |
 |  |  |  |  |
 2  3  4  5  6

[etc.]

However, before & after 4th Lateran there was some disagreement in the use of the "Roman" vs. the "Canon" or "Germanic" system of counting, as well as difference of opinion about how to count people descended in an unequal number of generations from a common ancestor. Many tables for use in measuring the degree of consanguinity survive in medieval manuscripts--often they are quite elaborate and beautiful. The best representative sample and study of the tables is by Hermann Schadt, Die Darstellungen der Arbores Consanguinitatis und der Arbores Affinitatis: Bildschemata in juristischen Handschriften (Tubingen, 1982).

The rules were supposed to apply equally to aristocrats and to humble people. Historians and genealogists see that the rules were often flouted in the case of aristocratic marriages. As Todd Farmerie has pointed out, they often married whoever suited them (economically, politically, etc.) and would only allow consanguinity to interfere if the marriage wasn't working out for other reasons. However, there are documented cases of churchmen intervening blocking some proposed marriages--even royal ones--against the desires of the parties themselves. Were these rules applied more stringently to persons of lower station? It isn't possible to tell, because we have virtually no documented examples of medieval non-aristocratic marriage in which the ancestries of the couple ore known fully, even in the fourth degree. The application and circumvention of consanguinity rules in non-aristocratic medieval marriages is a matter for speculation.

NB: The documented cases of bishops blocking proposed consanguineous marriages include two examples of apparent intended marriages of illegitimate daughters of Henry I. Saint Anselm intervened in the Warenne marriage, and Ivo of Chartres in another. Both are discussed by Bouchard, and by R. W. Southern, The Making of the Middle Ages (Yale, 1953), 79-80. Both involved relationships in the sixth degree. I've got the text of Ivo's letter buried somewhere in my office (his correspondence is in the PL).