Cousin

A system of degrees and removes is used to describe the relationship between the two cousins and the ancestor they have in common. The degree (first, second, third cousin, etc.) indicates the minimum number of generations between either cousin and the nearest common ancestor; the remove (once removed, twice removed, etc.) indicates the number of generations, if any, separating the two cousins from each other.

For example, a person with whom you share a grandparent (but not a parent) is a first cousin; someone with whom you share a great-grandparent (but not a grandparent) is a second cousin; and someone with whom you share a great-great-grandparent (but not a great-grandparent) is a third cousin. Someone with whom you share a great great great grandparent(but not a great great grand parent) is a fourth cousin. Someone with whom you share a great great great great grandparent (but not a great great great grandparent is a fifth cousin. Someone with whom you share a great great great great great grandparent (but not a great great great great grandparent) is a sixth cousin. The child of your first cousin is your first cousin once removed because the one generation separating you and the child represent one remove. You and the child are still considered first cousins, as your own grandparent (this child's great-grandparent), as the most recent common ancestor, represents one degree.

Non-genealogical usage often eliminates the degrees and removes, and refers to people with common ancestors merely as cousins or distant cousins.

The system can handle kinships going back any number of generations (subject to the genealogical information being available). In 2004, genealogists discovered that U.S. Presidential candidates George W. Bush and John Kerry shared a common ancestral couple in the 1500s. It was reported that the two men are sixteenth cousins, three times removed.^[1] However, the two are in fact ninth cousins, twice removed.^[2] Also, in 2007, it was revealed that U.S. vice president Dick Cheney and senator Barack Obama are eighth cousins.^[3]

If one goes back far enough, at some point all human beings will be found to be related according to the National Center for Biotechnology Information, an American institution.^[4]

Cousin chart

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. Cousinship between two individuals can be specifically described in degrees and removes by determining how close, generationally, the common ancestor is to each individual.

Additional modifying words are used to clarify the exact degree of relatedness between the two people. Ordinal numbers are used to specify the number of generations between individuals and a common ancestor, and further clarification of exact cousinship is made by specifying the difference in generational level between the two cousins, if any, by using degrees of remove. For example, "first cousins once removed" describes two individuals with the common ancestor being the grandparent of one cousin (one degree) and the great-grandparent of the other cousin. The cousins themselves are one generation different from each other (one remove).

If one person's ?		Grandparent	Great-grandparent	Great-great-grandparent	Great³-grandparent	Great⁴-grandparent	Great⁵-grandparent
is the other person's ?	then they are ?	Grandparent	Great-grandparent	Great-great-grandparent	Great³-grandparent	Great⁴-grandparent	Great⁵-grandparent
Grandparent		1st cousins	1st cousins once removed	1st cousins twice removed	1st cousins thrice removed	1st cousins four times removed	1st cousins five times removed
Great-grandparent		1st cousins once removed	2nd cousins	2nd cousins once removed	2nd cousins twice removed	2nd cousins thrice removed	2nd cousins four times removed
Great-great-grandparent		1st cousins twice removed	2nd cousins once removed	3rd cousins	3rd cousins once removed	3rd cousins twice removed	3rd cousins thrice removed
Great³-grandparent		1st cousins thrice removed	2nd cousins twice removed	3rd cousins once removed	4th cousins	4th cousins once removed	4th cousins twice removed
Great⁴-grandparent		1st cousins four times removed	2nd cousins thrice removed	3rd cousins twice removed	4th cousins once removed	5th cousins	5th cousins once removed
Great⁵-grandparent		1st cousins five times removed	2nd cousins four times removed	3rd cousins thrice removed	4th cousins twice removed	5th cousins once removed	6th cousins

Chart relationships as sentences

Reminder: the closest relationship prevails - note that cousinship is not calculated between individuals when one is descended from the other, for example, two individuals are not called cousins if they are any degree of grandparent, parent and child. Also cousinship is not calculated between individuals of any degree of aunt/uncle and nephew/niece relationship to each other.

Following this pattern, it can be determined that xth cousin y-times removed means either of the following:

Determining cousin type

The name of the cousinship is not determined by oneself, but rather is always determined by the generational level of the individual most closely related to the ancestor in common. The following assumes there are no double cousins:

An alternative method is as follows. You and your cousin count the generations between you and the common ancestor. Do not count the common ancestor and do not count yourselves. Thus, if it is a grand parent, this number is one. Let this be X. If X is different for the two of you, then let the difference between be Y. Now, use the smaller X (if there is a difference). You are X cousins, Y times removed. If Y is zero (because the number of generations between you and your ancestor is the same as for your cousin), then you are simply X cousins. X is stated as an ordinarial, i.e. first, second, etc.

Note that the above system is symmetric; if person A is person B's second cousin once removed, then person B is person A's second cousin once removed as well, even though the relationship between them is not symmetric (since the two are not from the same generation).

Also note that much of this terminology is variable; for example, many dictionaries give "a child of one's first cousin" as a secondary sense for the term second cousin (the primary sense being "a child of a first cousin of one's parent").

A different and partly conflicting system that is sometimes used is asymmetric (i.e. it mirrors the fact that aunt/uncle and niece/nephew are asymmetric names). With this system to work out what cousinage X is to Y, identify the descendant or ancestor of X that is the same generation as Y (i.e. the same number of generations from the common ancestor), then count how many generational removes there are up or down the tree from those same-generation cousins. In other words go across the family tree first, then up or down. For example take X and Y who have common ancestors who are X's great grandparents and Y's grandparents. From Y's point of view, X is Y's first cousin's child, and thus is Y's first cousin once removed (downwards), but from X's point of view Y's child is X's second cousin, and Y therefore is X's second cousin once removed (upwards).

Double cousins

Generally, one's cousinship to another is determined by a connection through only one parent's biological family. But an individual's cousinship to another individual may be determined by a connection through both of one's parents. These cousins are biologically connected to both the maternal and paternal family trees and that cousinship is termed a double cousin. Another term used to describe this is cousins on both sides.

If a pair of siblings from one family each form a couple with a pair of siblings from another family, then the children of these two couples will be double first cousins to one another. The children of the couples would already automatically be first cousins because they are children of one of their parent's siblings, but in this case the children of their mother's sibling, are also the children of their father's sibling, and thus they are double first cousins. Such cousins have double the consanguinity of ordinary cousins and are as related as half-siblings. Instead of the 12.5% consanguinity that simple first cousins share with each other, double first cousins share a 25% consanguinity with each other. Further, when identical twins form a coupling with a corresponding set of identical twins, the children of these two couples, though legally (double) first cousins to one another, would genetically be as closely related to each other as ordinary full siblings. When identical twins reproduce with the same person, the resulting children are likewise genetically indistinguishable from full siblings, although they are legally half-siblings AND first cousins. When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings. When two siblings who are not identical twins marry the same person, the resulting children are likewise more related than half-siblings but less related than full siblings. Children of double first cousins are double second cousins to each other.

Half cousins

Half-siblings share only one parent. Extrapolating from that, if one of John's parents and one of Mary's parents are half-siblings, then John and Mary are half-first cousins. The half-sibling of each of their respective parents would be their half-aunt or half-uncle but these terms, although technically specific, are rarely used in practise. While it would not be unusual to hear of another's half-brother, or half-sister, so described, in common usage one would rarely hear of another's half-cousins or half-aunt, so described, and instead hear them described simply as the other's cousin or aunt. And children of half-first cousins are half-second cousins to each other and so on because they would share only one common great-grandparent out of eight instead of two.

Mathematical definitions

The family relationship between two individuals a and b, where G_a and G_b respectively are the number of generations between each individual and their nearest common ancestor, can be calculated by the following:

Xth cousin y times removed means having x+1+ygenerations back in history and have the same ancestor. Xth cousin -y time removed means x+1 generations back in history and have the other go down x+1+y genrations So two people sharing a pair of grandparents have x = 2 and y = 0 and are described as being first cousins.

If x/v and they only share one nearest common ancestor rather than two, then the word "half" is sometimes added at the beginning of the relationship.

Granduncle/grandaunt and grandnephew/grandniece are also equivalent to great-uncle/great-aunt and great-nephew/great-niece. Both great-uncle and granduncle refer to an uncle of one's father or mother. Neither form is definitively more correct than the other. When "grand" is not used, the formula above becomes y − 1 greats.

The mathematical definition is more elegant if you always express consanguinity as the ordered pair of natural numbers (x, y) as defined above. In that case, the relationship one has with oneself is (0, 0), the relationship between parent and child is (0, 1), and the relationship between grandparent and grandchild is (0, 2). The relationship between siblings is (1, 0); and between aunt/uncle and nephew/niece is (1, 1). First cousins are (2, 0). The first number expresses how many generations back the two people's most recent common ancestor is, while the second number expresses the generation difference between the two people.

Alternative canon law charts

Another visual chart used in determining the legal relationship between two people who share a common ancestor (blood) is based upon a diamond shape, and is usually referred to as a canon law relationship chart.

The chart is used by placing the "Common Progenitor" (the 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 top most position and each child 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 respect 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.

Cousin

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