consanguinity & affinity

Very important concepts to know when searching catholic parish records,from the Facebook page Evidence Explained:
Consanguinity and Affinity:

We see the words often in religious records of the past and occasionally in civil ones. The concept of consanguinity--a blood relationship--is simple enough; but modern thinking often leads us astray where affinity is concerned. In civil records especially, we see references to "my brother" and other stated kinships that are hard to support by documentary evidence.

A deeper look at the historical meaning of the term can help our analyses. We borrow here from the online CATHOLIC ENCYCLOPEDIA (http://www.newadvent.org/cathen/01178a.htm).

"Affinity (in Canon Law)
"A relationship arising from the carnal intercourse of a man and a woman, sufficient for the generation of children, whereby the man becomes related to the woman's blood-relatives and the woman to the man's. I... Today affinity does not beget affinity. Therefore the relatives of the man do not become relatives of the woman's relatives, neither do those of the woman become relatives of the man's relatives. ...

"The motive for the impediment of affinity is akin to, though not as strong as, that of consanguinity; there arises ... a nearness and natural intimacy with the blood-relatives of the other side. The degrees of affinity are determined by the same rule as the degree of blood-relationship."

The essay offers considerably more insight. It's well worth reading.
http://www.newadvent.org/cathen/01178a.htm