Family tree

A cousin is a relative with whom a person shares one or more common ancestors. In the general sense, cousins are two or more generations away from any common ancestor, thus distinguishing a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew. However in common usage, “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”.
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
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).

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