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What is a Collateral Line in Genealogy?

What is a collateral line in genealogy? It is a line made of the descendants of your collateral ancestors, and in this post, learn all about these branches of our family tree.

You will discover:

  • The definition of a collateral line in a family tree
  • What a collateral ancestor really is
  • The difference between a collateral and direct ancestor
  • Whether collateral lines are important, and if we should add them to our family tree

I’m a huge fan of collateral lines in my family tree. They are the secret to busting through those brick walls, and learning even more about our ancestors.

Below, I’ll explain everything you have ever wanted to know about collateral lines and ancestors. By the end of this post, you will be double-checking to make sure you have added all of those collateral ancestors to your family tree.

What is the definition of a collateral line in a family tree?

A collateral line in a family tree, or genealogy, is the line of descendants of a collateral ancestor. These people are related to us genealogically and often times, genetically.

We are not descended from the collateral lines of our family tree. Relatives on our collateral lines are our first, second, third, fourth, fifth, and sixth cousins, et al.

They are all descended from our collateral ancestors. A good understanding of what a collateral ancestor is will help you truly understand collateral lines and why they are so important in genealogy.

Collateral lines are not in-laws, or ancestors of people in our tree who are related by marriage.

What is a collateral ancestor?

A collateral ancestor is someone who you are related to, but not directly descended from. We likely have collateral ancestors for almost every generation in our family tree.

For example, our aunts, uncles, and siblings of our grandparents are related to us. However, we are not descended from them, which makes them our collateral ancestors.

The further back we go in our tree, the more collateral ancestors we will have. This is because we will have twice as many ancestors as we move back through the generations in our tree.

Since most of our ancestors likely had siblings, we might end up with a great number of collateral ancestors. Including their descendants, which you now understand makes a collateral line, we can end up with a very big family tree.

What is the difference between a collateral and direct ancestor?

A collateral ancestor is someone who is descended from your common ancestor, but not a direct ancestor to you. Typically, a collateral ancestor is the sibling of your direct ancestor.

In the example we used before about aunts and uncles, we can say that your direct ancestor is your parent, and your collateral ancestor is the sibling of your direct ancestor.

We are directly descended from our parents and grandparents, which makes them our direct ancestors. This is the main difference between collateral and direct ancestors.

It can seem more complicated to think about this scenario when it comes to more distant ancestors. For example, imagine a fourth great-grandparent.

You are descended from a child of the great-great-great-great grandparent (4th great-grandparent). If your 4th great-grandparent had other children, siblings of your 3rd great-grandparent, they are your collateral ancestors.

Why are collateral lines important?

Collateral lines are very important for genealogy research. There is a very long list of ways in which collateral lines can help us with the study of our family tree, and below I have included my top reasons that our collateral ancestors and their descendants – the foundation of our collateral lines – are important.

Descendants of collateral ancestors may prove to be helpful research partners

People who are descended from our collateral ancestors are relatives to us, and many of them are also interested in family history. In fact, many of them may already be, or have been, involved in family tree research.

You might stumble upon the work of someone in your collateral lines while doing family tree research. This could range from an online family tree built on a site like Ancestry that includes information about some of your ancestors, or a book published long ago about a distant ancestor.

Every once in a while, you might find that one of these relatives is interested in working with you to solve a family tree mystery. Two minds are always better than one when it comes to genealogy, so definitely reach out to those collateral relatives when you find them.

Collateral ancestors may be the key to solving family tree mysteries

A few years ago, I was looking for information about the death of one of my third great-grandparents, and I was coming up with nothing. I spent hours upon hours trying to figure out where he died.

He didn’t seem to have lived in New Jersey with my great-great grandfather, his son, around the time of his death. Suddenly, I realized that he may have gone to live with one of my collateral ancestors in his last years, perhaps because he needed caring for as an elderly adult.

After a bit of research, I discovered that he had been living with one of his grand-daughters, who was a descendant of my collateral ancestor. In this case, my collateral ancestor was the sibling of my direct ancestor (my great-great grandfather).

It makes perfect sense that an elderly gentlemen would live with a kind granddaughter who was able to care for him. By researching my collateral line, I was able to track down my ancestor and discover information about his death that led me to discover the names of his parents.

Collateral lines are key to researching DNA matches

By researching our collateral lines, and adding our collateral ancestors and their descendants to our family tree, we can more easily discover how we are connected to our DNA matches.

If you have taken a DNA test, you might have noticed that you have a long list of DNA matches that are related to you in some way. However, their surnames, and those of their recent ancestors, might seem very unfamiliar.

Those people who diligently add their collateral lines to their family tree save a lot of time in their research of their DNA matches. They do not have to continually “rediscover” collateral lines, since they already know about them.

Coincidentally, the research strategy of adding collateral lines to a family tree is called building a “wide” family tree, and is an especially recommended strategy for those who have done DNA testing.

Should you add collateral lines to your family tree?

If you have read this article up until this point, you probably already know that you should definitely add collateral lines to your family tree. It will save you time and effort in the future, and it might help you solve some tricky genealogy mysteries.

We should add the siblings and descendants of our great-grandparents, great-great grandparents, great-great-great grandparents, and so on, to our trees. If we can, we should add their spouses and children as well.

I recommend adding as many people as you can to your family tree, except for distantly related living people. If you do decide to add living people to your family tree, whether they are closely or distantly related, be sure to always mark them as private in order to protect their personal information.

Upon occasion, I have found myself and other close family members listed in a distantly-related stranger’s family tree, usually without any privacy protection involved. So, in order to be a good family member, even to my distant relatives, I am sure to try to diligently protect their privacy and safety.


I hope that this post has helped you understand more about collateral lines, collateral ancestors, why learning about these people are important, and whether you should add them to your family tree.

If you have any questions about something that you have read in this post, I would love to hear from you in the discussion below.

Thanks for reading today!

Share the knowledge!

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Robert Shade

Wednesday 3rd of July 2024

Show examples of collateral family flow charts and best ways to add on family tree.Thanks

Don Bos

Sunday 7th of August 2022

Thanks for the new vocabulary.


Sunday 7th of August 2022

Your explanation of Collateral Lines is very helpful. I have been trying to identify a great grandfather using our family's DNA matches and have discovered that ThruLines identifies my suspected 4th GGF with 41 DNA matches. Would that pretty high number of matches indicate to me that I am on the right track? Actually, of all my ThruLines 4GGPs, 41 is the highest number of matches of them all, even 10 more than for several 4GGP's I definitely know are related. Should I also assume that my great grandfather could also be a different great grandson of this 4GGF? Thank you!

David Wilson

Friday 11th of March 2022

The information was very informative. I have made a bilateral and unilateral family trees for my book. Also, I have created what be called a collateral family tree, but it is not bilateral, rather I would say unilateral. For example, if I cover up my mother siblings an d follow the dingle link to my great-great grand parents it is like a unilateral trace. I would like to be able to name the tree based on its configuration. Should I call it a unilateral/collateral family tree? Any suggestions will be greatly appreciated. Thank you.


Thursday 17th of March 2022

Hi David, I like the "Collateral Family Tree" name, but I would definitely include an explanation of what that term means at the beginning of the book. Awesome idea, and best of luck to you! Sincerely, Mercedes

Carol Negri

Sunday 21st of March 2021

Is a half cousin from a collateral line?


Tuesday 23rd of March 2021

Hi Carol, Yes, a half-cousin is from a collateral line. The collateral line the line descended from your aunt or uncle, the parent of your half cousin. Excellent question!

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