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Family tree The family tree below shows the relationship of each person to the orange person, with cousins coloured 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 cousins", and "half 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 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. Double cousins and half 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 cousin. Another term 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. 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. Whether the first family siblings are brother and sister forming a coupling with a sister and brother of the second family, or whether the first family sibling pair are brothers forming a coupling with a pair of sisters from the second family, whenever two siblings from one family each form a couple with two siblings from another family, 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-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-cousins or half-aunt, so described, and instead hear them described simply as the other's cousin or aunt. Chart 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. 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. Similarly Similarly Following this pattern, it can be determined that xth cousin y-times removed means either of the following: Mathematical definitions 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| 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
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