I have a cool relative in the US. His grandmother and my grandmother are sisters.
What relation is that?
2nd cousins?
Yep.
Yes. Your parents (the children of the two grandmothers) were first cousins to each other. You are a first cousin, once removed, to your relative’s parent. Your relative is first cousin, once removed, to your parent.
And you’re second cousins to each other.
When you and your second cousin both have children, those children will be third cousins to each other. 
I presume that those two sisters were full sisters. That is, they had the same mother and father. If so, that mother and father would be your great-grandparents. If they only had one parent in common, you and your relative would be half-second cousins, not full second cousins.
People who share a parent are siblings. People who share a grandparent are cousins. People who share a great-grandparent are second cousins. People who share an Nth-great-grandparent are (N+1)th cousins.
Which means that siblings (with an ancestor of order 1 in common) are zeroth cousins, and you’re your own -1th cousin (as your own ancestor of order 0).
And any couple who’ve had kids are -2th cousins, because they have an ancestor of order -1 in common.
Sometimes the words are used that way, but to be more precise, people who share one parent are half-siblings and people who share two parents are siblings. The same is true for half-cousins and cousins in regards to grandparents. The same is true for half-second cousins and second cousins in regards to great-grandparents. And so on.
If your cool relative has a kid, then that kid is your second cousin once removed.
Not just any couple, but your parents, too! :eek:
Yes, everyone is the product of (negative-order) incest.
2nd Cousin.
I’ve got a 2nd cousin one on the other side of the country per 23andme. We can’t figure out how this is so but we are working on that.
Make a list of all your great-grandparents. Then send the list to your 2nd cousin and have them pick which one is also one of their great-grandparents.
Thanks for the replies.
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And if those second cousins decide to have children with one another, that child would be his own third cousin.
You had to go there, You just had to go there:smack:
*If you go to your family reunion, looking for a date…
…You might be a redneck*
They may not have the necessary information to tell.
My wife’s parents were first cousins once removed, so I guess she is her own second cousin once removed.
And yes, they were from one of the states where that is stereotypical (although her father had a PhD so they weren’t exactly hillbillies).
And no, they did not meet at a family reunion.
They met at a family funeral.
I don’t even want to get into the labeling in my family tree. My grandmother’s parents were first cousins, THEIR parents were first cousins.
And my grandmother’s sister married HER first cousin.
Cousin calculation works by finding closest common ancestor and noting how many generations removed from each person.
Call that x and y. The lower of the two numbers minus one is the cousin type. The difference between x and y is how much removed.
Example grandmas are sisters. So the two people have at the very least a common great grandparent. That is 3 generations removed so 3-1 is 2 and that makes a 2nd cousin. Since 2-2=0 it would be a 2nd cousin 0 removed.
The child of one of the people in the example would be 2nd cousin 1 removed to the other person in the example.
The always excellent CGP Grey has a videoon family trees.