From time to time I’ve noticed fellow bloggers expressing confusion about distant cousins. They will write something like “my second cousin twice removed (whatever that means).” As a professional historian who also assists with genealogical research, I am here to end your confusion.
People who share the same mother and/or father are brothers and sisters. People who do not share a parent but share at least one grandparent are first cousins. (Often, when we say “cousins,” we are referring to first cousins.) People who do not share any grandparents but share at least one great-grandparent are second cousins. People who do not share any great-grandparents but share at least one great-great-grandparent are third cousins. Tracing the human line back to Adam and Eve (or at least as far back as Noah), all people on earth are cousins to some degree, whether they are first cousins or thousandth cousins.
As for the distinction of “once removed” and so on: my first cousins’ children are my first cousins once removed. My first cousins’ grandchildren are my first cousins twice removed. My second cousins’ children are my second cousins once removed. My second cousins’ grandchildren are my second cousins twice removed. And so on. In other words, the levels of removal are differences in generation, even if (as is the case with me) you are closer in age to your first cousins once removed than you are to their parents, your first cousins.
The generational removal can go the other direction as well, but only if the kinship is not closer. For example, the parents of my first cousins are my uncle and my aunt, not my first cousins once removed. But, since the grandchildren of my first cousins are my first cousins twice removed, I am also their first cousin twice removed.
I hope this information is helpful. J.