The stemmata in ancient Roman houses (depicting portraits of ancestors) were used to assert the nobility of the nobles by right of family descent — stemmata distinguished between the patrician class (those with noble ancestry) and plebeians (commoners). It is therefore unsurprising that the Medieval nobility subsequently started to produce diagrams, as their way of illustrating their own succession in unambiguous terms (although it was not until much later that genealogies became common).
For example, as discussed in my post on The first royal pedigree, the earliest known illustration of a family tree is from c.1000 CE (see Schmid 1994), in which Cunigunde of Luxembourg's ancestry is traced in a tree-like manner to include the emperor Charlemagne (Charles the Great), thus legitimizing her claim to being of royal descent — she married Henry, Duke of Bavaria, in 999 CE, and he became King Henry II of Germany in 1002, at which point she became Queen consort of Germany (1002-1024).
However, pedigrees were also produced for the opposite purpose — to try to prevent marriages, for example on the basis that they violated church law. The earliest known such case involved the marriage, in 1043 CE, of King Henry III of Germany (1016-1056, later Emperor Heinrich of the Holy Roman Empire) to Agnès of Poitou (1025-1077).
Heinrich was briefly (1036-1038) married to Gunhilda of Denmark. After her death, for political reasons he wanted to remarry with someone from France. He chose the young daughter of Duke William V of Aquitaine. She thus became Queen consort of Germany (1043-1056) and then Empress consort of the Holy Roman Empire (1046-1056); from 1056-1061 she acted as regent of the Holy Roman Empire during the minority of her son Henry IV.
The official basis for objecting to this marriage was that the bride's and groom's maternal great-grandmothers were half-sisters, so that Henry and Agnes were third cousins. Moreover, on Henry's father's side they were also fourth cousins once removed. This is illustrated in the following genealogy from Michel Parisse (2004).
Note that Henry III appears twice, once as the son of his father and once as the son of his mother, thus simplifying the network to a tree; this is a point that I have commented on before.
The person formally objecting to this marriage was Siegried of Gorze, who researched the family history and drew the first version of the pedigree. As discussed by Bouchard (2001):
Abbot Siegried of the reformed monastery at Gorze wrote very shortly before [the marriage] to his friend Abbot Poppo of Stablo [or Stavelot], who possessed the confidence and respect of Henry, urging him even at the eleventh hour, and at risk of a possible loss of the king's favor, to do all that he possibly could to prevent it. Neither Poppo, nor Bishop Bruno of Toul (later Pope Leo IX), to whom Siegfried addresses still more severe reproaches, nor Henry himself, paid much heed to these representations.Henry apparently rebutted Siegried's claim by (falsely) claiming that the pedigree was at fault (ie. the great-grandmothers were not half-sisters). Nevertheless, various published versions of Siegfried's pedigree continued to appear over the subsequent 500 years (see Gädeke 1992). You can read Siegfried's original Latin letters (without the accompanying family tree) in the paper by Michel Parisse (2004). Jean-Baptiste Piggin has a transcription of the genealogy, taken from an early 11th century book (see the blog post: Two medieval drawings).
Part of the issue here is the change in the church laws relating to consanguinity (the degrees of relationship within which marriage was uncanonical), which had occurred during the first half of the ninth century. At that time, both the number of forbidden degrees was increased, from four to seven, and the method of calculating those degrees was changed. These two changes are illustrated here (from Bumke 1991).
So, the church councils held at Rome (during the first half of the eighth century) forbade marriage only between: siblings; parents and offspring; grandparents with grandchildren; a man and his niece (but not a woman and her nephew!); and first cousins. However, the canonical changes during the subsequent century forbade everything out to sixth cousin. The reasoning behind these extreme changes is not fully understood.
Needless to say, these new laws of consanguinity created an impossible situation when, as Bumke (1991) puts it:
in the course of the tenth and the first half of the eleventh century a small number of royal and princely families, already connected by marriage ties in the past, emerged and ruled most of western and central Europe.Under the new rules, it would not take long for a restricted group of people to become too closely related to inter-marry at all — royalty could not marry royalty. So, Henry set a precedent for his kin when he managed to bypass the new rules, which the aristocracy were likely to ignore anyway. These rules remained in force until 1215 (the Fourth Lateran Council), when the degrees were reduced again to four, but still counted in the "new" way.
As a final note, this sort of religious interference was not always unsuccessful. For example, in the early 1100s Henry I of England suggested marrying one of his (illegitimate) daughters to William de Warenne (2nd Earl of Surrey), but was dissuaded by Archbishop Anselm of Canterbury, who pointed out the prohibited degrees involved. Shortly afterwards, Bishop Ivo of Chartres successfully intervened in the proposed marriage of another of Henry's (illegitimate) daughters to Hugh fitz Gervaise of Châteauneuf-en-Thymerais.
Constance Brittain Bouchard (2001) Those of My Blood: Constructing Noble Families in Medieval Francia. University of Pennsylvania Press, Philadelphia.
Joachim Bumke (1991) Courtly Culture: Literature and Society in the High Middle Ages. University of California Press, Berkeley.
Nora Gädeke (1992) Zeugnisse bildlicher Darstellung der Nachkommenschaft Heinrichs I. Arbeiten zur Fruhmittelalterforschung 22. De Gruyter, Berlin.
Michel Parisse (2004) Sigefroid, abbé de Gorze, et le mariage du roi Henri III avec Agnès de Poitou (1043). Un aspect de la réforme lotharingienne. Revue du Nord 356-357: 543-566.
Karl Schmid (1994) Ein verlorenes Stemma Regum Franciae. Zugleich ein Beitrag zur Entstehung und Funktion karolingischer (Bild-)Genealogien in salisch-staufischer Zeit. Frühmittelalterliche Studien 28: 196-225.