From ISOGG Wiki
A cousin is a relative with whom a person shares one or more common ancestors. In the general sense, cousins are two or more generations away from any common ancestor, thus distinguishing a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew. Systems of "degrees" and "removals" are used in the English-speaking world to describe the exact relationship between two cousins (in the broad sense) and the ancestor they have in common. Various governmental entities have established systems for legal use that can more precisely specify kinships with common ancestors existing any number of generations in the past, though common usage often eliminates the degrees and removals and refers to people with common ancestry as simply "distant cousins" or "relatives".
The ordinals in the terms "first cousins", "second cousins", "third cousins", refer to the number of generations to one's closest common ancestor. The number of "G" words used to describe this ancestor will determine how close the relationship is. For example, having "Great-Great-Grandparents" in common would be third cousins. When the cousins are not the same generation, they are described as "removed". In this case, the smaller number of generations to the common ancestor is used to determine the degree, and the difference in generations determines the number of times removed. Note that the ages of the cousins are irrelevant to the definition of the cousin relationship.
The degree (first, second, third cousin, et cetera) indicates one less than the minimum number of generations between both cousins and the nearest common ancestor. For example, a person with whom one shares a grandparent (but not a parent) is a first cousin; someone with whom one shares a great-grandparent (but not a grandparent) is a second cousin; and someone with whom one shares a great-great-grandparent (but not a great-grandparent) is a third cousin; and so on.
The remove (once removed, twice removed, etc.) indicates the number of generations, if any, separating the two cousins from each other. The child of one's first cousin is one's first cousin once removed because the one generation separation represents one remove. Oneself and the child are still considered first cousins, as one's grandparent (this child's great-grandparent), as the most recent common ancestor, represents one degree. Equally the child of one's great (also known as "grand")-aunt or uncle (who is one's parent's cousin) is one's first cousin once removed because their grandparent (one's own great-grandparent) is the most recent common ancestor.
Non-genealogical usage often eliminates the degrees and removes, and refers to people with common ancestors merely as cousins or distant cousins.
A cousin chart, or table of consanguinity, is helpful in identifying the degree of cousin relationship between two individuals using their most recent common ancestor as the reference point.
For an alternative view see also the cousin and family relationship chart provided by Crestleaf.com. For background see the Crestleaf article Are you my relative? Family relationship and infographic
There is a mathematical way to identify the degree of cousinship shared by two individuals. In the description of each individual's relationship to the most recent common ancestor, each "great" or "grand" has a numerical value of 1. The following examples demonstrate how this is applied.
Example 1: If person one's great-great-great-grandfather is person two's grandfather, then person one's "number" is 4 (great + great + great + grand = 4) and person two's "number" is 1 (grand = 1). The smaller of the two numbers is the degree of cousinship. The two people in this example are first cousins. The difference between the two people's "numbers" is the degree of removal. In this case, the two people are thrice (4 − 1 = 3) removed, making them first cousins three times removed.
Example 2: If someone's great-great-great-grandparent (great + great + great + grand = 4) is another person's great-great-great-grandparent (great + great + great + grand = 4), then the two people are 4th cousins. There is no degree of removal because they are on the same generational level (4 − 4 = 0).
Example 3: If one person's great-grandparent (great + grand = 2) is a second person's great-great-great-great-great-grandparent (great + great + great + great + great + grand = 6), then the two are second cousins four times removed. The first person's "number" (2) is the lower, making them second cousins. The difference between the two numbers is 4 (6 − 2 = 4), which is the degree of removal (generational difference).
A niece or nephew could be referred to as a "zeroth cousin once removed", and thus a sibling as a "zeroth cousin (zero times removed)". This can be extended to define oneself as a "minus one cousin", parents and children as "minus one cousins once removed", and so on. This forms the basis of an inductive definition of "Nth cousin M-times removed".
- "Genetic And Quantitative Aspects Of Genealogy - Types Of Collateral Relationships". Genetic-genealogy.co.uk. http://www.genetic-genealogy.co.uk/Toc115570138.html#Single_Cousins. Retrieved 28 October 2014.
- Average British person has 193,000 living cousins says new research. Daily Mirror, 17 June 2015.
- Henry Louis Gates and Julie Granka. The interconnectedness of the human family. Ancestry blog, 8 April 2015.
- Mills ES. The kinship maze: navigating it with professional precision. Association of Professional Genealogists Quarterly 20 (June 2005): 61-70. Digital image available at Elizabeth Shown Mills’ Historic Pathways website at: http://historicpathways.com/download/navkinmaze.pdf. An excellent article which defines the basic terminology from a variety of different perspectives (legal, anthropological, genealogical, etc).
- Types of collateral relationships An article from the Genetic and Quantitative Aspects of Genealogy website.
- Go ahead, kiss your cousin by Richard Coniff, Discover magazine, 1 August 2003.
- Cousin couples A website with information on cousin marriages.
- A list of coupled cousins on Wikipedia.
- Cousin charts and explanations from Paul Stoneburner
- Genealogy relationship chart from About.com
- Cousin relationship calculator from Ancestor Search
- Cousin calculator Free download