Consanguinity and Medieval Marriages

Eleanor of Aquitaine

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The term “consanguinity” simply means how close a blood relationship two persons have—how recently they have a common ancestor.

Ancient History

In Egypt, brother-sister marriages were common within the royal family. If the Biblical stories are taken as history, Abraham married his (half-)sister Sarah. But such close marriages have been generally prohibited in cultures from fairly early times.

Roman Catholic Europe

In Roman Catholic Europe, the canon law of the church forbid marriages within a certain degree of kinship. Which relationships were forbidden to marry varied at different times. While there were some regional disagreements, until the 13th century, the church forbade marriages with consanguinity or affinity (kinship by marriage) to the seventh degree—a rule which covered a very large percentage of marriages.

The pope had the power to waive the impediments for particular couples. Frequently, papal dispensations waived the block for royal marriages, especially when more distant relationships were generally forbidden.

In a few cases, blanket dispensations were given by culture. For example, Paul III restricted marriage to the second degree only for American Indians and for natives of the Philippines.

Roman Scheme of Consanguinity

Roman civil law generally prohibited marriages within four degrees of consanguinity. Early Christian custom adopted some of these definitions and limits, though the extent of prohibition varied somewhat from culture to culture.

In the Roman system of calculating the degree of consanguinity, degrees are as follows:

  • The first degree of kinship includes: parents and children (direct line)
  • The second degree of kinship includes: brothers and sisters; grandparents and grandchildren (direct line)
  • The third degree of kinship includes: uncles/aunts and nieces/nephews; great-grandchildren and great-grandparents (direct line)
  • The fourth degree of kinship includes: first cousins (children sharing a pair of common grandparents); great uncles/great aunts and grand nephews/grand nieces; great grandchildren and great grandparents
  • The fifth degree of kinship includes: first cousins once removed; great grand nephews/great grand nieces and great grand uncles/great grand aunts
  • The sixth degree of kinship includes: second cousins; first cousins twice removed
  • The seventh degree of kinship includes: second cousins once removed; first cousins three times removed
  • The eighth degree of kinship includes: third cousins; second cousins twice removed; first cousins four times removed

Collateral Consanguinity

Collateral consanguinity—sometimes called Germanic consanguinity—adopted by Pope Alexander II in the 11th century, changed this to defining the degree as the number of generations removed from the common ancestor (not counting the ancestor). Innocent III in 1215 restricted the impediment to the fourth degree, since tracing more distant ancestry was often difficult or impossible.

  • The first degree would include parents and children
  • First cousins would be within the second degree, as are uncle/aunt and niece/nephew
  • Second cousins would be within the third degree
  • Third cousins would be within the fourth degree

Double Consanguinity

Double consanguinity arises when there is consanguinity from two sources. For example, in many royal marriages in medieval times, two siblings in one family married siblings from another. The children of these couples became double first cousins. If they married, the marriage would count as a first cousin marriage, but genetically, the couple had closer connections than first cousins who were not doubled.


These rules about consanguinity and marriage were developed before genetic relationships and the concept of shared DNA were known. Beyond the genetic closeness of second cousins, the statistical likelihood of sharing genetic factors is almost the same as with unrelated individuals.

Here are some examples from medieval history:

  1. Robert II of France married Bertha, a widow of Odo I of Blois, in about 997, who was his first cousin, but the Pope (then Gregory V) declared the marriage invalid and eventually Robert agreed. He tried to get an annulment of his marriage to his next wife, Constance, to remarry Bertha, but the Pope (by then Sergius IV) would not agree.
  2. Urraca of Leon and Castile, a rare medieval reigning queen, was married in her second marriage to Alfonso I of Aragon. She was able to get the marriage annulled on the grounds of consanguinity.
  3. Eleanor of Aquitaine was married first to Louis VII of France. Their annulment was on the grounds of consanguinity, fourth cousins descended from Richard II of Burgundy and his wife, Constance of Arles. She immediately married Henry Plantagenet, who was also her fourth cousin, descended from the same Richard II of Burgundy and Constance of Arles. Henry and Eleanor were also half-third cousins through another common ancestor, Ermengard of Anjou, so she was actually more closely related to her second husband.
  4. After Louis VII divorced Eleanor of Aquitaine on grounds of consanguinity, he married Constance of Castile to whom he was more closely related, as they were second cousins.
  5. Berenguela of Castile married Alfonso IX of Leon in 1197, and the Pope excommunicated them the next year on grounds of consanguinity. They had five children before the marriage was dissolved; she returned to her father’s court with the children.
  6. Edward I and his second wife, Margaret of France, were first cousins once removed.
  7. Isabella I of Castile and Ferdinand II of Aragon—the famous Ferdinand and Isabella of Spain—were second cousins, both descended from John I of Castile and Eleanor of Aragon.
  8. Anne Neville was a first cousin once removed of her husband, Richard III of England.
  9. Henry VIII was related to all of his wives through common descent from Edward I, a fairly distant degree of kinship. Several of them were also related to him through descent from Edward III.
  10. As just one example from the multiply-intermarried Habsburgs, Philip II of Spain married four times. Three wives were closely related to him. His first wife, Maria Manuela, was his double first cousin. His second wife, Mary I of England, was his double first cousin once removed. His third wife, Elizabeth Valois, was more distantly related. His fourth wife, Anna of Austria, was his niece (his sister’s son) as well as his first cousin once removed (her father was Philip’s paternal first cousin).
  11. Mary II and William III of England were first cousins.
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Your Citation
Lewis, Jone Johnson. "Consanguinity and Medieval Marriages." ThoughtCo, Feb. 16, 2021, Lewis, Jone Johnson. (2021, February 16). Consanguinity and Medieval Marriages. Retrieved from Lewis, Jone Johnson. "Consanguinity and Medieval Marriages." ThoughtCo. (accessed June 2, 2023).