Have you any idea who a “second cousin, once removed” is? I am indebted to a friend, Bill Liberi; as a result of his curiosity and research, I think I now understand. Let’s see if I can pass it on.
The definition of a first cousin is something most seem to grasp readily: You are the first cousin of the child of any aunt or uncle (who is the sibling of either of your parents). You share two of four grandparents with each of your first cousins.
When we use the term second cousin (or third, fourth, etc.), we are describing people who are at the same generational level in relation to their ancestors. The same is true for third cousins, fourth cousins, etc.; they must always be at the same generational level.
So, if A is the mother and B and C are her children, B and C represent the same generation. (Throughout, when we refer to generations in these illustrations, it will always be relative to A.) If B and C have children, B1 and C1, respectively, B1 and C1 are first cousins. Again, they are of the same generation.
Next B1 and C1 each has a child, B2 and C2, respectively; B2 and C2 are again of the same generation (great grandchildren of A) and, in relationship to each other, they are second cousins. If B2 and C2 have children, B3 and C3, B3 and C3 are of the same generation (great, great grandchildren of A), and they are third cousins. This continues indefinitely. (For the sake of simplicity and clarity, each of our alphabet people has only one child, except for A who has two. Real life is never that simple.)
People at different generational levels (such as uncle and niece) can never be first, second, third cousins, except by adding appendages, such as in once removed, twice removed, or thrice removed which are used to designate relatives of different generations.
For purposes, of clarity, even though it may be repetitious, in our illustration A is first generation; B and C are second generation; B1 and C1 are third generation; B2 and C2 are fourth generation; and so it goes. If the little numbers of the alphabet people are the same, they are of the same generation.
Using only alphabet people from above, B and C are first cousins (the children of siblings B and C). B1’s child, B2, and C1 are still considered to be first cousins, but now they are first cousins, once removed (i.e., B2 is a generation below C1). When B2 has a child, B3, the relationship of “first cousin” between C1 and B3 is preserved (because B3 is directly descended from B1 who is C1’s first cousin); but because B3 is now two generations below C1, they are first cousins, twice removed.
Now you have the principles; putting our alphabet people to work again, who would be second cousins, once removed? We know that B2 and C2 are both of the fourth generation, and we know (from above) that they are second cousins; going to the next generation then we find that C3 (fifth generation) and B2 (fourth generation) are second cousins, once removed, as are B3 and C2.
Continuing, C4 is the second cousin, twice removed of B2.
Because family lineage tends to spread out very quickly with each generation, it can be challenging, but less so if you create a chart. (Imagine my family: My parents had eight children; 27 grandchildren; and, so far, 29 great grandchildren and counting!). Go back up a generation or two in your own family, chart it, and see where it leads; who is what in relation to others. (You did not hear me say anything about money-back guarantees.)
— Ken Butera