Whether they’re part of the same “generation”
Suppose I have a cousin A. I have a child B, and A has a child C.
B and C are second cousins. B and A are first cousins once removed. C and I are first cousins once removed.
The number of “removes” is how many unequal generations apart you are. My first cousin’s child is my first cousin once removed, my first cousin’s grandchild is my first cousin twice removed, etc.
The number of “cousins” (first cousins, second cousins, etc) is how many equal generations apart you are. So the children of first cousins are second cousins to each other. The children of second cousins are third cousins to each other, though relationships past second cousins are rarely relevant to real life.
First cousin – One of my grandparents is also one of your grandparents.
Second cousin – One of my great grandparents is also one of your great grandparents (but we don’t share a grandparent).
First cousin once removed – One of my grandparents is one of your great grandparents, or vice versa. (“Once removed” means that you’re one generation apart)
Count the number of generations back to the first common ancestor for both “cousin.” Use the shorter distance to figure first-second-third. Same great grandparent, second cousin. Same grandparent, first cousin. Same parent, well, that’s your sibling (zeroeth cousin?).
If it’s a different number of generations on each side, the difference is the number of times removed.
Abe’s great grandparent is Betty’s grandparent, they’re first cousins once removed.
Charlie’s great grandparent is Dottie’s great great grandparent, they’re second cousins once removed.
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