A cousin in kinship terminology is a relative with whom one shares a common ancestor, but in modern usage the term is rarely used when referring to a relative in one's own line of descent, or where there is a more specific term to describe the relationship, e.g., brother, sister, aunt, uncle. The term blood relative can be used synonymously, and underlines the existence of a genetic link.
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; and so on. 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. However, the two are in fact ninth cousins, twice removed. Also, in 2007, it was revealed that U.S. vice president Dick Cheney and senator Barack Obama are eighth cousins.
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, part of the American National Institutes of Health.
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||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|
---- Following this pattern, it can be determined that xth cousin y-times removed means either of the following:
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).
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.
Chart relationships as sentences
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 practice. 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.
Xth cousin y times removed means having x+1+y generations 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.
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.