How Are We Related?

Terminology:

Common Progenitor:

The closest ancestor two people have in common is their common progenitor.
For example: you and your sister have your parents as your common progenitor.  (You also have your grandparents and great-grandparents in common, but for these purposes, we are concerned only with your closest common ancestor). 
Another example: the common progenitor of you and your first cousin is one of your grandparents.

Removed:

When we speak of a cousin being “once removed”, we are referring to the number of generations removed.
For example: Your father's first cousin is your “first cousin once removed” — You are one generation away (“removed”) from the first-cousin relationship.
Another example: Your grandfather's first cousin is your “first cousin twice removed” — You are two generations away from the first-cousin relationship.

Relationship Chart:

The numbers on the top and on the left side represent generations.  To show how two people are related, we must first figure out who is the common progenitor (the "CP", the closest ancestor which two people have in common — which might be a parent, grand-parent, great grand-parent, etc.)  After you have determined who the common progenitor is, then figure out how many generations for one and then the other, and then go to the point where they intersect — that is your relationship.

For example, suppose you want to know the relationship between yourself and your first cousin's son.  The first question to ask is: Who is the closest ancestor to both of us?  The answer is your grandfather (or grandmother, but for simplicity, the chart shows only male descent, though it is the same for both males and females).

In the left column, notice that the square next to number 2 says "GS", which stands for grandson.  That is you (in our example).  On the row across the top, you can see that the square below number 3 says "GGS", which means great-grandson.  That is your first cousin's son.  Again, your grandfather and your first cousin's great-grandfather are the same person.  On the chart, you are number 2 and he is number 3.

The square at which row 2 and column 3 meet tells you the relationship.  That is, the square which says "1C1R" — which means “first cousin once removed”.  That is your relationship to each other.

Cousins:

Your first cousins are the people in your family who have two of the same grandparents as you.  In other words, they are the children of your aunts and uncles.

Your second cousins are the people in your family who have the same great-grandparents as you, but not the same grandparents.

Your third cousins have the same great-great-grandparents, fourth cousins have the same great-great-great-grandparents, and so on.

The word “removed” indicates that the two people are from different generations; it tells you the number of generations away from the first-cousin relationship.

Can you be your own cousin?  Yes.  If, for example, your grandparents were first cousins when they married, then you are your own third cousin!

More Information:

For additional information, see:

• Wikipedia article: Cousin