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In genealogy, a cousin is a relative, other than an ancestor or descendant, who shares a common ancestor. The term cousin is often used more specifically, to refer to a first cousin, the child of one's aunt or uncle. Even when the more general sense is used, the term is rarely applied to siblings, siblings of ancestors, or descendants of siblings, because more common terms are in use for such relations (e.g., brother, sister, aunt, uncle, nephew, niece).
A cousin chart, or table of consanguinity, is a chart that identifies cousin relationships using a most recent common ancestor as a reference point. Since all humans share a common ancestor, any two individuals have a cousin relationship — except if one of the individuals' parents is a common ancestor, in which case a different set of terminology — mother, niece, uncle, brother and so on — is used. Note that this chart implicitly ranks relationships based on shared ancestry, a ranking that might not apply to all cultures. Some cultures draw a strong distinction between adoptive and biological parents, or between older and younger siblings; neither distinction is reflected here, though even in English such complexities can be conveyed using clarifying adjectives and other modifiers. Conversely, some cultures do not distinguish between blood relations and relations by marriage; this chart does not even include the latter.
This family tree diagram shows the relationship of each person to the orange person, with cousins colored in green. When referring to someone as a cousin, additional modifying words are used to clarify the exact relationship between the two people. First and second cousins for instance, use ordinal numbers to specify the number of generations between individuals and a common ancestor. Examples of additional modifying phrases include "first cousins once removed", "double-first cousins", and "half-first cousins".
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. Cousinship is actually a description of three individual's relationships with each other. Oneself, the cousin, and the ancestor in common. The following assumes there are no double-first cousins:
Step 1: To work out if two people are first, second, or third cousins, count back the generations to their common ancestor. For example, if the common ancestor is one's grandmother, that is two generations. If it is one's great-grandmother, that is three generations.
Step 2: Take the closest descendant of the common ancestor. For example, if one of the cousins is a great-great-grandchild (four generations) and the other is a grandchild, just consider the grandchild for now.
Step 3: If the closest descendant of the common ancestor is a grandchild (two generations), the cousins are first cousins; if three generations, second cousins, and so on.
Step 4: If the cousins are both separated from the common ancestor by the same number of generations, there is no "removed". If the number of generations from the common ancestor is different, that difference is the number of "removed"s.
For example, if one is a grandchild of (2 generations from) the common ancestor, and one's cousin is a great-great-grandchild of (4 generations from) the common ancestor, then one and one's cousin are first cousins twice removed.
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").
Double-first cousins and half-first cousins
Generally, one's cousinship to another is determined by a connection through only one parent to an individual in that parent's biological family. But an individual's cousinship to another individual may be determined by a connection through both parents. These cousins are biologically connected to both the maternal and paternal family trees and that cousinship is termed a double-first cousin. Another term is cousins on both sides. Such cousins have double the consanguinity of ordinary cousins and are as related as half-siblings.
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. They would already automatically be first cousins due to the fact that 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.
Whenever two siblings from one family each form a couple with two siblings from another family (regardless of the gender of any of the siblings), the offspring of these two couples will be double first cousins to one another. Instead of the 12.5% consanguinity that first cousins share with each other, double first cousins share a 25% consanguinity with each other. If identical twins form a coupling with a corresponding member of another 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 full siblings of a non-twin set of parents would be to each other.
Sometimes the children of these unions would be called cousin-siblings, cousin-brothers, or cousin-sisters.
Note that no incest has occurred to create these close kinships.
Half-siblings share only one parent. Extrapolating, 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 though technically correct 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-first cousins or half-aunt, so described, and instead hear them described simply as the other's cousin or aunt. Half cousins would share 6.25% consanguinity.
Half First cousins share 1 grandparent. First Cousins share 2 grandparents. Double First Cousins share 4 grandparents. If a set of siblings has children with a set of haf siblings the cousins would share 3 grandparents and their consaguinity would be 18.75%.
The chart below helps explain cousin relationships.
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.
|If one person's →||Grandparent||Great-grandparent||Great-great-grandparent||Great-great-great-grandparent||Great-great-great-great-grandparent||Great-great-great-great-great-grandparent|
|is the other person's
|then they're ↘|
|Grandparent||First cousins||first cousins once removed||first cousins twice removed||first cousins thrice removed||first cousins four times removed||first cousins five times removed|
|Great-grandparent||first cousins once removed||Second cousins||Second cousins once removed||Second cousins twice removed||Second cousins thrice removed||Second cousins four times removed|
|Great-great-grandparent||first cousins twice removed||Second cousins once removed||third cousins||third cousins once removed||third cousins twice removed||third cousins thrice removed|
|Great-great-great-grandparent||first cousins thrice removed||Second cousins twice removed||third cousins once removed||fourth cousins||fourth cousins once removed||fourth cousins twice removed|
|Great-great-great-great-grandparent||first cousins four times removed||Second cousins thrice removed||third cousins twice removed||fourth cousins once removed||fifth cousins||fifth cousins once removed|
|Great-great-great-great-great-grandparent||first cousins five times removed||Second cousins four times removed||third cousins thrice removed||fourth cousins twice removed||fifth cousins once removed||Sixth 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.
- If our parents are siblings we are first cousins, and have the same grandparents
- If our grandparents are siblings we are second cousins and have the same great-grandparents
- If our great-grandparents are siblings we are third cousins and have the same great-great-grandparents
- If our great-great-grandparents are siblings we are fourth cousins and have the same great-great-great-grandparents
- My first cousin's child and I are first cousins once removed to each other (one generation difference between us)
- My first cousin's grandchild and I are first cousins twice removed to each other (two generations difference between us)
- My parent's first cousin and I are first cousins once removed to each other (one generation difference between us)
- My grandparent's first cousin and I are first cousins twice removed to each other (two generations difference between us)
- My second cousin's child and I are second cousins once removed to each other (one generation difference between us)
- My second cousin's grandchild and I are second cousins twice removed to each other (two generations difference between us)
- My parent's second cousin and I are second cousins once removed to each other (one generation difference between us)
- My grandparent's second cousin and I are second cousins twice removed to each other (two generations difference between us)
Following this pattern, it can be determined that xth cousin y-times removed means either of the following:
- The xth cousin of your direct ancestor y generations previously (eg. your great-grandparent's fifth cousin is your fifth cousin thrice removed); or
- Your xth cousin's direct descendant y generations away (eg. your fifth cousin's great-grandchild is also your fifth cousin thrice removed)
The family relationship between two individuals a and b, where Ga and Gb respectively are the number of generations between each individual and their nearest common ancestor, can be calculated by the following:
- x = min (Ga,Gb)
- y = |Ga-Gb|
- If x=0 and y=0 then they are the same person.
- If x=0 and y=1 then they are parent and child.
- If x=0 and y=2 then they are grandparent and grandchild.
- If x=0 and y>2 then they are great ... great-grandparent and great ... great-grandchild, with y−2 greats.
- If x=1 and y=0 then they are siblings (brothers or sisters).
- If x=1 and y=1 then they are uncle/aunt and nephew/niece.
- If x=1 and y>1 then they are great ... great uncle/aunt and great ... great nephew/niece, with y−1 greats.
- If x>1 and y=0 then they are (x−1)th cousins.
- If x>1 and y>0 then they are (x−1)th cousins y times removed.
So two people sharing a pair of grandparents have x=2 and y=0 and are described as being first cousins.
If x>0 and they only share one nearest common ancestor rather than two, then the word "half" is sometimes added at the beginning of the relationship.
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 which both people are descended) in the top space within 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 the 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 wanted to use the chart to determine their relationship using the chart to the right, their common parents would be 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 would 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.
- Kinship terminology
- Cousin couple
- Family members
- Most recent common ancestor
- Parallel cousin
- Cross cousin
- Genealogical numbering systems
- Double first cousin
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