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 thrice removed.
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).
Example 3: If one person's great-grandparent (great + grand = 2) is a second person's great-great-great-great-great-grandparent (great + great + great + great + great + grand = 6), then the two are second cousins four times removed. The first person's "number" (2) is the lower, making them second cousins. The difference between the two numbers is 4 (6 − 2 = 4), which is the degree of removal (generational difference).
Read more about this topic: Cousin
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