'It's a wise child that knows its own father,' is an old saying that indicates the complexities that can occur in relationships. However, if Robert is your mother's brother then you can say, unequivocally, Bob's your uncle.
The church and the state have set out many rules about who you are allowed to marry and who you are not. There is much discussion about the closeness of relationship (consanguinity) and where you stand in the law. In other words, if you are too closely related you cannot marry, and any such relationship is considered incestuous.
Strangely enough, rules on this distance of relationship are not set in stone. In the United States about half the states, mainly in the South and East, allow first cousins to marry.
The Christian churches have to be careful about how they describe the taboo on close relationships because of the creation story which begins with Adam and Eve. By definition, Adam and Eve's children (brothers and sisters to each other) had to 'marry' if they were to continue the race.
Other significant biblical figures married close relatives - Abraham married Sarah, who was a child of his father through a different mother (ie, his half sister)1, and in Exodus 6:20, we are told that Amram took Jochabed, his father's sister (aunt), to wife, and she bore him Aaron and Moses.
The church and state have often had different interpretations of what constituted an illegal marriage, and you will need to look elsewhere for what your local rules are. Despite previous laws Elizabeth I said that 'no prohibition, God's law except, shall trouble or impeach any marriage outside Levitical law'. This meant that marriages between cousins, which had been illegal prior to her reign, were now legal. Over the years the church has changed its ruling on marriage to bring it more into line with the state.
If you want to find out what your formal relationship is with another member of your family, the closest common ancestor needs to be found - this person is the closest common ancestor you both descend from. You never go beyond that person when working out your relationship2. When you can both say what your relationship is to the common ancestor, you can describe your relationship to each other.
Let's call the common ancestor Audrey. If you both call Audrey 'mummy' then you are siblings - ie, sister/sister, brother/brother or brother/sister. If the closest common ancestor is more than one generation away then you need to read further.
If you both call Audrey 'grandmother' then you are first cousins. If you both call Audrey 'great grandmother' you are second cousins. This system keeps working for each generation. Great-great grandchildren are third cousins and so on.Common Ancestor - Relationship
Mother3 - Siblings
Grandmother - Cousins
Great-Grandmother - Second Cousins
Great-Great-Grandmother - Third Cousins
Simple isn't it!
You may have heard of being a first cousin once removed or a third cousin twice removed and wondered what all that meant and why all these removals. It is not much more complicated.
Of the two of you who are trying to work out your kinship, the one nearest the common ancestor dictates whether you are first cousins, second cousins and so on. The other person dictates the level of removal. So if you find that Audrey is the closest common ancestor and you call her grandmother while the other person (let's call him Dennis) calls her great grandmother, you are the closer.
Grandchildren, as explained above, are 'first cousins' (normally just the term cousin is used), so you are a first cousin in some way to Dennis. Dennis calls Audrey great grandmother, so that makes him one generation further 'removed' from Audrey. This then completes the picture. You are first cousins once removed.
|Audrey (Your Grandmother)
| Brian, your Father
|(Brothers)|| Bob, Dennis's Grandfather
|You||(Cousins)||Carl, Dennis's Father
|(One generation removed from yours)||Dennis|
Handily for us, in your family you have an unusual family tradition. Everyone born into the family in Audrey's generation was given a name beginning with the letter A, her children were all given names beginning with B, and the grandchildren were given names beginning with C. The tradition continues in this way, with each new generation taking names beginning with the next letter of the alphabet.
At a family wedding you call Audrey 'grandmother', and there is a woman, Doris, who refers to her as 'great grandmother'. You know Doris is not your daughter or your niece, so she is the daughter of one of your cousins. This makes Doris your first cousin once removed - and you are her first cousin once removed4.
Your own son Daniel comes in and says, 'Hello, great grandmother,' to Audrey. He gives Audrey the same title Doris does, so he and Doris are of the same generation - even if there is a big age gap - and he and Doris are second cousins.
To your amazement Doris introduces a boy to Audrey and tells her that this is Emir, Audrey's great-great grandson. Emir is your first cousin twice removed and Daniel's second cousin once removed.
When working out relationships you have to find out the closest common ancestor from whom you and your relative have descended. You must both refer to that individual with a title that has mother or father as part of it (grandmother, great grandfather, great-great grandmother and so on).
Now, suppose you call the closest common ancestor 'mother' or 'father' and your relative is not a sibling or descendant of yours. You are the uncle (or aunt) of the other, with the same number of 'greats' in your title as your mother or father has. So you call the common ancestor 'mother' and your relative calls her 'grandmother' - no 'greats' in this title so no 'greats' in yours; you are just an uncle or aunt. If the relative calls your mother 'great grandmother' you are their great uncle or great aunt.
Why can't people just be simple?
Naming people alphabetically by generation would make life so much simpler. But in reality people don't stick to such rules. In some families, where there has been incest, one person can be the father and grandfather of the same child. People re-pair and have second and third sets of children with different partners. These children have half brothers and sisters (sometimes without realising it, if the previous relationship was kept a secret). This then brings in half-cousins. Here the cousins share a common ancestor but the partner of that ancestor is different for each branch of the family. In consanguinity a half cousin is closer than a second cousin - but you probably knew that.
BEWARE People often refer to people as cousins when they are not. Find out how many generations there are back to a common ancestor, who is a parent to both your and their branches of the family. You can then work things out methodically with paper and pencil.