With the holiday season at our throats once again, we thought it would be a good time to explain how family relationships work. This simple guide will help you through any seasonal gathering.
Basically, first, second, ..., nth cousins have common ancestors n+1 generations ago. First cousins have common grandparents ... two generations back. Second cousins have common great grandparents, three generations back. This is illustrated in the following diagram.
As you can see from this, siblings (common parents) are really 0th cousins. Grandparents, then, would be -1st cousins, and so forth back through the generations.
Once removed means one level of generation skew. Twice removed means two levels, and so forth.
If Grandchild D and Grandchild E are cousins, then Grandchild D and Great Grandchild K are first cousins once removed. Your first cousin's children are your first cousins once removed. (But they're your children's second cousins.) Your first cousins' grandchildren are your first cousins twice removed (and your children's second cousins once removed, and your children's children's third cousins.) Of course, this means that Child A and Grandchild E are 0th cousins once removed, or siblings once removed. We'll leave that as an exercise for the reader.
See? It's easy? In a future post, we'll talk about special relativity.
Special thanks to Ken Davis, my 0th cousin.