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A cousin is a relative with whom a person shares one or more common ancestor(s) (other than a parent, child/descendant, sibling, child/descendant of a sibling, or sibling of a parent/ancestor). However in common parlance, "cousin" normally specifically means "first cousin".
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".
Basic definitions 
The ordinals in the terms "first cousins", "second cousins", "third cousins", describe the "degree" of the cousin relationship. The degree of two cousins' relationship is determined by the number of generations to their closest common ancestor. 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 children of two siblings.
David and Emma are first cousins because they are non-siblings who share the same grandparents in common.
The children of two first cousins.
Frank and Gwen are second cousins because they are non-first cousins who share great-grandparents in common.
The grandchildren of two first cousins; also the children of two second cousins.
Harry and Isabel are third cousins because they are non-siblings who share great-great-grandparents in common.
First cousins once removed
Two people for whom a first cousin relationship is one generation removed.
Frank and his father's first cousin, Emma, are first cousins once removed.
First cousins twice removed
Two people for whom a first cousin relationship is two generations removed.
Harry and his grandfather's first cousin, Emma, are first cousins twice removed.
Second cousins once removed
Two people for whom a second cousin relationship is one generation removed.
Harry and his father's second cousin, Gwen, are second cousins once removed.
Additional terms 
The following is a list of less common cousin terms.
|Double cousin||Double cousins arise when two siblings of one family reproduce with two siblings of another family. The resulting children are related to each other through both of their parents, and are thus doubly related. Double first cousins share both sets of grandparents in common and have twice the degree of consanguinity of ordinary first cousins. Double second cousins can arise in two ways: from two first-cousin relationships among their parents, or from one double-first-cousin relationship between their parents.||David and Irene are double first cousins because each is related through their mother's family and also their father's family, the result of a brother and sister (Ben and Helen) having married another brother and sister (Hugh and Betty). For David and Irene, each has a mother who is an aunt-in-law of the other, and a father who is an uncle-in-law of the other.||
|Half-cousin||Half cousins are the children of two half siblings.||David and Lilian are half cousins because their fathers (Ben and James) are half brothers, their grandmother (Agatha) having divorced and remarried.||
|Step-cousin||Step-cousins are either stepchildren of an individual's aunt or uncle, or nieces and nephews of one's step-parent.||David and Mary are step-cousins because David's uncle (Charles) has become Mary's stepfather as a result of Mary's mother (Corinda) having divorced and remarried.||
|Cousin-in-law||A cousin-in-law is the spouse of an individual's cousin, or the cousin of one's spouse.||David and Edward are first cousins-in-law to each other, as Edward's wife (Emma) is David's first cousin.||
|Maternal or Paternal cousin||A term that specifies whether one individual is a cousin of another through the mother's side of the family (maternal) or the father's side (paternal). If the relationship is not equally paternal for both or equally maternal for both, then the paternal cousin of one is the maternal cousin of the other.||Emma and David are paternal first cousins (being related through their fathers). Emma is also Nicola's paternal first cousin (as related on Nicola's father's side), but Nicola is Emma's maternal first cousin (as related on Emma's mother's side). David and Nicola are not related to each other.||
Kissing cousins are defined by the Oxford English Dictionary as "relatives or friends with whom one is on close enough terms to greet with a kiss".
Relationship charts 
Cousin chart 
|This section does not cite any references or sources. (February 2011)|
A "cousin chart", or "table of consanguinity", is helpful in identifying the degree of cousin relationship between two people using their most recent common ancestor as the reference point. Cousinship between two people can be specifically described in degrees and removals by determining how close, generationally, the common ancestor is to each person.
|If one person's →||Grandparent||Great-grandparent||Great-great-grandparent||Great-great-great-grandparent||Great-great-great-great-grandparent|
|is the other person's
|then they are ↘|
|Grandparent||1st cousins||1st cousins once removed||1st cousins twice removed||1st cousins thrice removed||1st cousins four times removed|
|Great-grandparent||1st cousins once removed||2nd cousins||2nd cousins once removed||2nd cousins twice removed||2nd cousins thrice removed|
|Great-great-grandparent||1st cousins twice removed||2nd cousins once removed||3rd cousins||3rd cousins once removed||3rd cousins twice removed|
|Great-great-great-grandparent||1st cousins thrice removed||2nd cousins twice removed||3rd cousins once removed||4th cousins||4th cousins once removed|
|Great-great-great-great-grandparent||1st cousins four times removed||2nd cousins thrice removed||3rd cousins twice removed||4th cousins once removed||5th cousins|
Canon law relationship chart 
Another visual chart used in determining the legal relationship between two people who share a common ancestor is based upon a diamond shape, usually referred to as a "canon law relationship chart".
The chart is used by placing the "common progenitor" (the most recent person from whom both people are descended) in the top space in the diamond-shaped chart, and then following each line down the outside edge of the chart. Upon reaching the final place along the opposing outside edge for each person, the relationship is then determined by following that line inward to the point where the lines intersect. The information contained in the common "intersection" defines the relationship.
For a simple example, in the illustration to the right, if two siblings use the chart to determine their relationship, their common parents are placed in the topmost position and each child is assigned the space below and along the outside of the chart. Then, following the spaces inward, the two would meet in the "brother/sister" diamond. If their children want to determine their relationship, they would follow the path established by their parents, but descend an additional step below along the outside of the chart (showing that they are grandchildren of the common progenitor); following their respective lines inward, they would come to rest in the space marked "1st cousin". In cases where one side descends the outside of the diamond further than the other side because of additional generations removed from the common progenitor, following the lines inward shows both the cousin rank (1st cousin, 2nd cousin) plus the number of times (generations) "removed".
In the example provided at the right, generations one (child) through ten (8th great-grandchild) from the common progenitor are provided; however, the format of the chart can easily be expanded to accommodate any number of generations needed to resolve the question of relationship.
Mathematical definitions 
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: 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).
Alternative definitions 
Asymmetric definitions 
The definitions discussed in the article above are the ones found in dictionaries of standard English, but they are not universal. At least one alternative usage also exists.
In this alternative system, the degree of the relationship from cousin A to cousin B is determined by the distance from A to the common ancestor and the number of times removed is the difference in generations between A to B. Sometimes "upwards" or "downwards" is used to indicate the direction of this difference. For example, if A has a grandparent whose sibling is B's parent, then B is A's "first cousin, once removed (upwards)", whereas A is B's "first cousin once removed (downwards)". Note that this is not standard terminology, and is completely absent from many major dictionaries.
As seen in this example, this usage is asymmetric, since different terms are used to represent A's relationship to B and B's relationship to A. By contrast, the standard usage of "cousin" discussed in the main part of this article is symmetric (in this example, the standard terminology would be that A is B's first cousin once removed, and B is A's first cousin once removed), and is also dyadic (for example, one can say "A and B are first cousins once removed").
Colloquial usage 
In day to day speech, "cousin" is often used unmodified. Normally it means a first cousin, but some people use the term "cousin" to refer to cousins of all types, such as first, second, and third cousins, as well as cousins once or more times removed. Modifier terms such as "half-cousin" or "step-cousin" are rarely used in everyday speech.
Usage for extremely distant relations 
Although use of the word "cousin" in this context is infrequent (especially outside of evolutionary literature), any two individual organisms regardless of their respective species (or any other level of taxonomy) are in fact very distant cousins by virtue of shared descent from a single cell whose descendants survived beyond the Paleoarchean Era.
See also 
- Collateral descendant
- Cousin marriage
- Cross cousin
- Genealogical numbering systems
- Kinship terminology
- List of coupled cousins
- Most recent common ancestor
- Parallel cousin
- Fitzhugh, Terrick V H (1988 (5th edition)). The Dictionary of Genealogy. London: A & C Black. p. 81. ISBN 0-7136-4859-7.
- kissing cousins Oxford English Dictionary. Retrieved: 2012-11-22.
- Merriam Webster dictionary, c.v. "second cousin".
- Fasold, Ralph and Shuy, Roger, Studies in Language Variation ISBN 0-87840-209-8
- Theobald, D. L. (2010), "A formal test of the theory of universal common ancestry", Nature 465 (7295): 219–22, Bibcode:2010Natur.465..219T, doi:10.1038/nature09014, PMID 20463738
- Hesman Saey, T. (14 May 2010). "All Modern Life on Earth Derived from Common Ancestor". Discovery News.
|Look up cousin in Wiktionary, the free dictionary.|