One of my pet peeves is a term that I see online over and over: someone claiming to be a “half first cousin” or a “half second cousin once removed” or something similar. Sorry folks, but there is no such thing as a “half first cousin” according to legal dictionaries. However, the term is used by others. I know that lots of families use that term to refer to various relatives.
NOTE: I will describe references used in the U.S. It is possible that relations are described differently in other countries and especially in languages other than English.
Many people think that a “half first cousin” is someone who shares one grandparent with you but not both of them. For instance, my great-grandfather was married twice. He had several children by his first wife. The wife then died in childbirth, and great-grandfather later remarried and had more children by his second wife. I am descended from great-grandfather and his first wife. I recently met a man who is descended from my great-grandfather and his second wife. Some people would think that this other man and I are half-second cousins. “Half” apparently refers to the fact that we share only half the relationship because of our different great-grandmothers.
In fact, we are second cousins. Period.
In the United States, the standard reference for defining family relationships is Black’s Law Dictionary. It is primarily a legal reference and is used by courts, lawyers, genealogical organizations, and many others.
Black’s Law Dictionary defines first cousins as:
“The children of one’s aunt or uncle.” Note that it says “aunt OR uncle,” not aunt and uncle. All that is required is to share one aunt or one uncle, not both.
Black’s Law Dictionary defines second cousins as:
“Persons who are related to each other by descending from the same great-grandfather or great-grandmother.” Note that it says “the same great-grandfather OR great-grandmother,” it does not say BOTH great-grandparents. Second cousins need to share only one great-grandparent. If they share both great-grandparents, the relationship doesn’t change; they are still second cousins.
Source citation: you can see an image of the appropriate page from Black’s Law Dictionary at http://blacks.worldfreemansociety.org/2/C/c0293.jpg. That image is from an old version of Black’s Law Dictionary but the definition hasn’t changed since then.
Canon law (religious law, usually referring to the Catholic church, but other religions also have rules that may be referred to as Canon law) agrees with Black’s legal definitions.
However, not all the experts agree. Elizabeth Shown Mills has tackled the problem in an article published in the Association of Professional Genealogists Quarterly, June 2005 in Navigating the Kinship Maze. She points out that the term “half cousin” does not appear in legal dictionaries but is commonly used by geneticists and others. Elizabeth doesn’t make any claims as to what is correct or incorrect. Instead, she describes the use of the word by different groups, including legal, forensic, genetic, anthropological, and religious perspectives. She also adds in some words found in legal documents but not commonly found in genealogy, including cognatic descent, and cousin german (which has nothing to do with Germany).
All of these sources seem to agree on one thing: the terms great-uncle and grand-uncle are interchangeable. The only difference is local usage.
If anyone is interested in learning more about relationships, I strongly suggest reading Elizabeth Shown Mills’ article, Navigating the Kinship Maze, available in the Association of Professional Genealogists Quarterly, June 2005 at http://historicpathways.com/download/navkinmaze.pdf.