This has nothing to do with Einstein’s theory of E=mc2. Instead it’s a quick guide aimed at those who are new to genealogy and who may be confused by some of the terms that keep cropping up. For example, what’s the precise relationship between you and your 5th cousin 2xremoved?
Most people know that their first cousins share with them at least one common grandparent. With 2nd cousins you have to go back to a great-grandparent to find the common link, and for 3rd cousins to a great-great grandparent. The rule is to count up the number of "g"s (both "great"s and “grand”). So 5th cousins would share a great-great-great-great-grandparent (usually expressed as a 4xgreat-grandparent or 4th great-grandparent). That’s 4 "great"s plus 1 “grand” = 5 "g"s. The formula works when you’re looking at two people of the same generation.
But what if, when you count up, one of you has more "g"s than the other? Well that just means that that person is one generation more "removed" from the common ancestor, i.e. he or she is a cousin "once removed".
In other words “removed” means that the two of you are of different generations. Take a look at the following chart showing the immediate ancestry of the three founders of this website, Martin Blackett, Pat Longbottom & Al Kirtley.
Working back, the most recent common ancestor of both (a) Pat and (b) Martin’s father, Eric Blackett, is Joseph Blackett (1844-1934). He’s their great-grandfather (i.e. 2 "g"s) and they were therefore 2nd cousins. Martin, though, is one generation further removed from that common ancestor, so he’s Pat’s 2nd cousin 1xremoved and vice versa. To find the relationship to Al you have to go all the way back to Cuthbert Blackett (1745-1809). He’s the 4xgreat-grandfather of Al, Pat and Eric Blackett. Al is therefore a 5th cousin of Pat & Eric (4x"great"s + one “grand”), and a 5th cousin 1xremoved of Martin.
So what relation is Martin to Pat’s children? Well he’s the same generation as them so there’s no “removed”. You have, though, to go back to THEIR great-great-grandfather, Joseph Blacket (1844-1934) to get to their joint ancestor. That’s 2 "great"s + 1 “grand” = 3 "g"s and so they’re 3rd cousins. If you do the same thing with Al’s children you have to go right back to their 5xgreat-grandfather, Cuthbert Blackett (1745-1809), to find the joint ancestor they share with Martin (or with Pat’s children). That’s 5 "great"s + 1 “grand” = 6 "g"s and they’re 6th cousins.
Another example is Al’s relationship to Pat and Martin’s ancestor Joseph Blackett (1806-1870). If you work back up through Al’s ancestors to the same generation as Joseph you get to Robert Blackett (1816-1903). Robert and Joseph were 1st cousins (same grandfather). Al is 4 generations removed from Robert, so he’s Joseph’s 1st cousin 4xremoved. The relationship applies both ways, i.e. Joseph is Al’s 1st cousin 4xremoved.
With uncles and aunts it’s a bit more straightforward. Your father’s uncle is your great-uncle, (sometimes written as two separate non-hyphenated words), your grandfather’s uncle is your great-great-uncle and so on. The only difference is that in North America many (though not all) people use “grand-uncle” instead of “great-uncle” and that’s often how it shows up in the Relationship Calculator function in most genealogy computer programs. So a “great-great-uncle” would be called a “great grand-uncle”. In the UK and Commonwealth countries, however, “grand” is rarely used. (Some people may remember “Great Uncle Bulgaria” in the TV animated series, The Wombles.)
The golden rule, however, is to check with your Relationship Calculator.