While it is possible to refer to a number of extended family members simply as "cousins," the precise relationship can get more complicated than that. This is why there are terms like "first cousins," "second cousins," and "first cousins once removed." What do all these terms mean?
First Cousins Share a Grandparent
The easiest way to understand the relationships between cousins is with a hypothetical example. This illustration keeps the family tree as simple as possible, ignoring most siblings and the extended families of spouses who have married into the reference family depicted here.
The closest common ancestor between first cousins is a grandparent. In the example above, your maternal grandmother and grandfather had two children: your mom and Danny. Your mom went on to marry your dad and have you. Danny went on to marry Pam to have Michelle.
Second Cousins Share a Great-Grandparent
Understanding second cousins is much the same, except the family tree must go back one further generation.
While first cousins have a grandparent as their closest common ancestor, second cousins have a great-grandparent as their closest common ancestor.
In the above extended example, your grandmother has a brother who is your great-uncle. The two of them were born to your great-grandmother and your great-grandfather.
Here, your great-uncle (sometimes referred to as a grand-uncle) married your great-aunt (sometimes referred to as a grand-aunt) and they had a daughter named Vivian. In turn, Vivian married Philip and had a daughter named Ashley. With this hypothetical example, you and Ashley are second cousins. This is because the closest common ancestor that you share is your great-grandma.
What Does "Once Removed" Mean?
In addition to the distinction between "first" cousins and "second cousins," there is also the added designation of being "once removed" or "twice removed." This refers to the number of generations that separate the two relatives in question and it is added on after it is noted that they are first cousins, second cousins, and so on.
Here, it is important to note that deciding whether you are "first" cousins or "second cousins" is relative to the person who is closest in generation to the common ancestor. For instance, the relationship between you and Vivian in the above chart is that you are first cousins, once removed.
This is because the common ancestor is your great-grandma, who is actually Vivian's grandmother. She is closer on the family tree to the common ancestor. Then, from a generational perspective, there is one generation that separates you and Vivian, as she is on the same level as your mother and your Uncle Danny.
Extending the Family Relations
The terminology used to describe the relationship between cousins can be further extended to reflect as many generations as needed.
First cousins share a common grandparent, second cousins share a common great-grandparent, third cousins share a common great-great-grandparent (the grandparent of a grandparent), and so on. Similarly, cousins who are once removed are one generation apart, cousins who are twice removed are two generations apart, and cousins who are thrice removed are three generations apart.
It's all relative. Indeed, it was revealed in 2007 that Dick Cheney and Barack Obama are actually related - they're eighth cousins, albeit only by marriage.