Genealogy 101: Cousins

It seems a bit mysterious how to calculate relations beyond the usual, (just what is a "second cousin"?) The following is really helpful.

In kinship terminology, a cousin is a relative with whom one shares a common ancestor. 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, et cetera) indicates one less than the minimum number of generations between both cousins and the nearest common ancestor. For example, a person with whom one shares a grandparent (but not a parent) is a first cousin; someone with whom one shares a great-grandparent (but not a grandparent) is a second cousin; and someone with whom one shares a great-great-grandparent (but not a great-grandparent) is a third cousin; and so on.

The remove (once removed, twice removed, etc.) indicates the number of generations, if any, separating the two cousins from each other. The child of one's first cousin is one's first cousin once removed because the one generation separation represents one remove. Oneself and the child are still considered first cousins, as one's grandparent (this child's great-grandparent), as the most recent common ancestor, represents one degree. Equally the child of one's great-aunt or uncle (one's parent's cousin) is one's first cousin once removed because their grandparent (one's own great-grandparent) is the most recent common ancestor.

Non-genealogical usage often eliminates the degrees and removes, and refers to people with common ancestors merely as cousins or distant cousins. Alternatively, the terms 'second cousin' and 'first cousin once removed' are often incorrectly used interchangeably.

The system can handle kinships going back any number of generations (subject to the genealogical information being available).

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