Ancestry/Genealogy

Let Fred and Ethel be cousins of some degree.

Count backward the fewest number of generations to find their common ancestor. If that number is 2, they are first cousins; if 3, they are second cousins; and so on.

Now count the number of generations from that common ancestor to Fred, and the number from that common ancestor to Ethel. Then calculate the difference between those two numbers. If that difference is 0, they are simply cousins, be they first cousins, second cousins, etc. If that difference is 1, they are once removed, be they first cousins once removed, second cousins once removed, etc. If that difference is 2, they are twice removed, be they first cousins twice removed, second cousins twice removed, etc. And so on.

Example: Fred has a great-great-great-great-grandmother named Lucy, who is also Ethel's great-great-grandmother. Counting generations backward from Fred to Lucy, that's 6 generations (great[1] + great[2] + great[3] + great[4] + grand[5] + mother[6]), while counting backward from Ethel to Lucy, it's 4 generations (great[1] + great[2] + grand[3] + mother[4]). 4 is less than 6, so we use 4 generations to determine the degree of cousin, and that means third cousins. Then we look at the difference between 4 and 6 generations; the difference is 2, so the cousins are twice removed. Therefore, Fred and Ethel are third cousins twice removed to each other.