For some reason there is often a problem in describing cousin relationships, at times I manage to confuse myself when trying to explain these relationships to others.
In essence relationships are easy to describe in words but do get more confusing the further away they are. Let us start at the beginning with someone you know – yourself.
Everyone has two parents, no more and no less. Current research may make this fact obsolete as it may be possible, in the future, for someone to have three parents, and of course I am only referring to biological parents.
These two parents may have brothers and sisters, these are your aunts and uncles. Any children of your parents siblings are usually called your cousins, technically they are your first cousins, and you all share the same grandparents. These grandparents are known as your most recent common ancestors (MRCA). Your mothers parents are the MRCA of you and your mothers siblings children, you have a different MRCA with your fathers side of the family.All of your Grandparents may have had siblings, they are your Great Aunts and Great Uncles, their children are first cousins of your parents and their siblings, any children of the first cousins of your parents are your second cousins, with one of your sets of Great Grandparents as MRCA. Perhaps better explained with a picture.
It does go on the same way for more generations, as seen here. The MRCA of any cousins are the parents of the first siblings you get to.
So that is the easy bit, where people often have more problems is in generation differences, the removed references. So lets look at these, if you have a first cousin then you have a set of Grandparents in common, but what of your cousins children? They have the same couple as common ancestors with you but for them they are Great Grandparents, you cannot be first cousins as the MRCA are not Grandparents for both of you, and the same logic excludes them from being second cousins, you are in fact first cousins once removed, there is one generation step extra in one of the lines. Similarly when they have children they will be your first cousins twice removed as they have two generation steps more than you to get to the MRCA. It is all really quite simple.
The important first step is finding the MRCA for the two people. If the MCRA are parents then the people are siblings, if only one parent is common then they are half siblings. If Grandparents are the MRCA then they are first cousins, Great Grandparents gives 2nd cousins and so on, the cousin number is the number of Greats plus one. So if you have a 6 times Great Grandparent in common with someone then you are 7th cousins. The person with the shortest number of steps to the MRCA determines the cousin number and the difference in number of steps determines the number of removes.
Understanding cousin relationships becomes more important with DNA research, often relationship matches are listed in the form of 2nd Cousin Once Removed to 2nd Cousin Twice Removed. This tells you the DNA match is possibly with your or his Great Grandparents but there is a one or two generation step in one of the lines, so my Grandparents, their parents of Grandparents should be the MRCA. All my lines around that time are fully researched in the same area as his family lived so all he needs to do is find his ancestors back to 2 X Great Grandparents and we will have a match on paper as well as DNA. So far this has produced no matches in eight months of research mainly due to undocumented illegitimate births and cuckolded fathers.