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What is a Third Cousin?

What is a third cousin? Are third cousins genetically related to you? How many third cousins do you have? Find answers to these questions and more in this post.

Most of us don’t grow up knowing our third cousins very well, unless we live in places where we have lots of distant extended family living all around us. Only a small percentage of us live in small towns where we are probably related in, in some way, to almost everyone.

What is a Third Cousin_

The term “third cousin” can be confusing, since many of us are not exactly sure what it really means. This is why it is no surprise that people want to know what is a third cousin and exactly how they are connected on their family tree.

Note: You will find that a genealogical third cousin is often different than a third cousin DNA match: What is a Third Cousin DNA Match?

What is a third cousin?

What is the meaning of third cousin? Your third cousin is the child of your parent’s second cousin and share great-great grandparents as their most recent common ancestor.

The parents of third cousins are related, too. Your parent and your third cousin’s parent are second cousins.

Third cousins share at least one great-great grandparent, and are descended from different offspring of those great-great grandparents. To put it another way, your great-grandparent and your third cousin’s great-grandparent were siblings.

For many people, the easiest way to understand how third cousins are related to each other: third cousins are the children of second cousins. In other words, the parents of third cousins are second cousins.

Furthermore, the grandparents of third cousins are first cousins.

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Example of how third cousins are related

Sometimes it is easier to understand a visual explanation about how 3rd cousins are connected. It can be tough to keep all of the greats and the great-greats in order in our head!

In the image below, we can see see descendants of Peter and Earlene. By studying their descendants, you will be better able to understand the definition of third cousins.

What is a third cousin?  This image is a visual example of third cousins., beginning with siblings (children of the great-grandparents) and ending with the third cousins
In this image, you can see how the descendants of Peter and Earlene are related to each other. Four generations down from Peter and Earlene are third cousins whose great-grandparents were siblings.

As you can see, the children of Peter and Earlene are siblings, their grandchildren are first cousins, their great-grandchildren are second cousins, and their great-great-grandchildren are third cousins.

This is essentially an answer to the other very common question: “If our great-grandparents were siblings, what are we?” If your great-grandparents were siblings, it means that your most recent common ancestors are the parents of your great-grandparents, which means you are 3rd cousins.

Are third cousins considered distant cousins?

Third cousins are not technically considered to be distant relatives. A distant relative is someone who is more distantly related than a third cousin.

By definition, a third cousin is not a distant cousin. All of our cousins who are related to us at a third cousin distance or closer are defined as close cousins.

Are third cousins considered family?

One possible criteria that people may use to determine whether they consider someone to be a close or distant relative may be whether they knew their common ancestor personally.

As you now know, third cousins share great-great grandparents, but their parents shared great-grandparents. Since the second cousins (the parents of the third cousins) may have known their common ancestor personally (their great-grandparents), they are more likely to raise their children to know their more extended family.

I have a few second cousins on my dad’s side of the family who I knew fairly well growing up. They have their own children now, as do I, which means that our children are 3rd cousins to each other.

While geography separates us, I expect that someday soon our children will meet each other. They will play and have fun, and hopefully, grow up with fond memories of their third cousins.

Plus, they will have the added benefit of understanding what third cousins are, which is great.

What is a third cousin once-removed?

A third cousin once-removed is the child of your third cousin or the third cousin of your parent. In other words, your parents‘ third cousins and the children of your third cousins are all your third cousins once-removed.

Cousins removed” can be a confusing concept, especially if you are new to the idea. It’s not too difficult to calculate how whether you are third cousins or third cousins “removed”, however.

First, make sure that you are really third cousins (and not more closely related). Is the person most closely descended from the common ancestor a great-great grandchild of the ancestor?

Then, calculate how many generations the other person (i.e. you, or the other cousin) is from the MRCA (most recent common ancestor), and subtract four.

For example, if you are six generations away from the most recent common ancestor, you would subtract six from four (the number generations the third cousin is from the MRCA). The answer (two) is the number of generations removed you are from your third cousin.

In this example, you and your cousin are third cousins twice-removed.

What is a half-third cousin?

A half-third cousin is technically a third cousin who shares only one great-great grandparent with you instead of two. This type of situation occurs most frequently when the great-great grandparent’s spouse remarried after the death or divorce of their first spouse.

If the great-great grandparent had a child with their first spouse and then has a child with their second spouse, their children are half-siblings. Their grandchildren would be half-first cousins, their great-grandchildren half-second cousins, and finally, their great-great grandchildren would be half- third cousins.

How many third cousins do I have?

On average, we can expect to have about 190 third cousins. This is assuming that the average family has 2-3 children and those children grow up to have 2-3 children, etc.

The exact number of third cousins that you actually have will depend on how many children all of your great-great grandparents had and how many survived to adulthood and began their own families, as well as:

  • cultural traditions related to family size
  • political and economic situations

It would be very difficult to know the names of all of your third cousins, much less meet all of them personally. However, since third cousins are not considered to be distant cousins, it is a worthy goal to at least know who all of them are.

There are two excellent ways to discover how many third cousins you have. First, you could take an Ancestry DNA test to receive a list of genetic relatives and their estimated relationship to you.

You could also start building your family tree (check out my book on Amazon on how to do it). Since our third cousins are descended from our great-great grandparents, we need to build a family tree.

It’s free to build a family tree on Ancestry, which is where I always build my trees. If you want to get a subscription to make things a little easier, I recommend it.

Are third cousins blood related?

Third cousins are always considered to be relatives from a genealogical perspective, and there is about a 90% chance that third cousins will share DNA.

With that said, third cousins who do share DNA only share an average of .78% of their DNA with each other, according to 23andMe. The exact amount of shared DNA between third cousins can vary, but it is rare for third cousins to share more than 2%.

Is it okay to date your third cousin?

Since third cousins share only a very small percentage of their DNA, there is no issue with third cousins dating from a genetic perspective.

Dating is one thing, but can you marry your third cousin?

According to an article by The Spruce, marriage between second cousins and more distant cousins is legal throughout the United States. There are some very famous third cousin marriages, such as the union between Queen Elizabeth II and the Duke of Edinburgh, both of the United Kingdom.

Queen Elizabeth and “Prince Phillip”, as he was otherwise known, were third cousins. As you know, third cousins share the same great-great grandparents.

The common ancestor shared between Queen Elizabeth and her husband, her third cousin, was Queen Victoria. Queen Elizabeth is directly descended from Queen Victoria’s heir, Edward VII, while Prince Phillip was descended from Queen Victoria’s second oldest daughter, Princess Alice.

Other famous third cousin marriages include John Adams, second president of the United States, and his 3rd cousin, Abigail Smith. There is even some evidence that Thomas Jefferson was third cousin to his wife, Martha Wayles Skelton.

I was unable to find a country in the world that has a legal prohibition to third cousin marriages, as there is really no good reason to prevent them. In many communities, third cousins may marry without even knowing that they are third cousins.

Not everyone knows who their great-great grandparents were, after all!

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I hope that this post has helped you understand more about third cousins, exactly how you are related, and how many third cousins you might have out there in the world.

If you have any questions about something that you read in this post, or if you would like to share your own experience learning about your third cousins, I would love to hear from you in the discussion below.

Thanks for stopping by today – you are awesome!

Share the knowledge!

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Maria krohn

Tuesday 2nd of November 2021

My son has a third cousin so what am I to the third cousin


Thursday 4th of November 2021

Hi Maria, You are second cousins once-removed to your son's third cousin. This is because you are one generation closer to your shared common ancestor, your son's great-great grandparent (i.e. your great-grandparent). I hope this helps!

Jasmine Johnson

Sunday 8th of August 2021

If my grandma and his grandpa are sibling how are we related?


Sunday 8th of August 2021

Hi Jasmine! Thank you for your question. If your grandmother and his grandfather are siblings, then your most recent common ancestors are your great-grandparents, which means that you and your relative are second cousins. You can learn more about second cousins here: I hope this helps you :) Sincerely, Mercedes

James Scobbie

Thursday 15th of July 2021

Hiya, I found this post hoping to find a quick and easy answer to the question of how many 3rd cousins there are in a tree in which everyone has 2 (or 3) children, and each child has 2 (or 3) in turn, etc. I calculate for 2 children (a binary tree), half of the tree can have 3rd cousin relationships with the other half, so that's 8x8=64 3rd cousins (each half of the tree has 8 people). But for 3 children (a ternary tree), there are three groups each with 27 people, so there are 27x27 3rd cousins, times 3 - that's 2,187. I've often seen people who get DNA done reporting very large numbers of 3rd or 4th cousins - not the estimate of 190 on average you mention - is it an empirical number gleaned from experience, rather than a bit of maths, or have I got my maths wrong?


Saturday 17th of July 2021

Hi James, Thank you so much for your question - very interesting! I should add a section about that to this post to account for different family sizes. You bring up a very interesting point about 3rd cousin DNA matches. Some of the "third cousin" DNA matches are actually third cousins in a genealogical sense, but others might be second cousins, half-second cousins, third cousins once or twice-removed, double-fourth cousins, etc. The DNA testing companies estimate our relationship to our matches based on our amount of shared DNA, which means that they will put people who share DNA that falls within that range into the third cousin category whether we are actually third cousins. This post goes into what a third cousin match really is:

John Goff

Thursday 22nd of April 2021

Maybe this is a little technical, but here goes. "K" and I are said to be 4th cousins by 23andMe. Our trees have one common name, so we are led to look for a common ancestor there. But there is none. Question is, is it reasonable to look in the next older generation, i.e. 5th cousin relationship (4th gr-grandparents)? I know the probability of detecting a match at this distance is small, but the probability might be enhanced since the 4th cousin match failed.


Tuesday 23rd of February 2021

I looked up and read this article because Seth Meyers included a joke pertaining to Rudy Giuliani having married his second cousin. I have quite a fractured family personally, and certainly don't know any other than the most immediate of cousins. My father even had a sister he never knew about till his mother's death (my grandmother). So I was interested and intrigued to wonder whether I might ever have known a second or third cousin and not even recognise them due to genetic variance. This article went a ways to explaining the relation ships of second third etc, as well some primers so i can continue seeking this information out of sheer interest. So a "Thank you!" to those who spent their time and effort making this information available. Cheers!


Tuesday 23rd of February 2021

Thank you so much for your comment! I'm glad you found the article useful and interesting, and I enjoyed your story about what inspired you to search for this information. Sincerely, Mercedes

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