DNA Triangulation

Can’t figure out how you’re related to your DNA match? Want to use DNA matches to identify an unknown ancestor? Try triangulation. Here’s how.

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I always loved math, but not geometry. Too many complicated theorems and formulas to remember. But, as a parent, you get to go through school all over again, even the parts you didn’t really like. This second time around, I was a little more in-tune with the triangle formulas because of an ongoing conversation we have about triangulation in genetic genealogy.

What is triangulation?

However, it seems that triangles are inherently complicated, as even genealogists seem to have two different definitions about what the term means. One camp says that triangulation occurs when three or more people share the same DNA segment. Another says it is triangulation simply when three people are all sharing DNA - even if it is not on the same place in the DNA. In this article, we will be talking about the second kind, just shared DNA, without worrying about where that DNA is shared.

Triangulation in this sense is all about building a genetic network out of lots of these three-person triangles. You can do this if you’ve tested at one of the major genetic genealogy testing sites: 23andMe, AncestryDNA, Family Tree DNA or MyHeritageDNA.  

DNA triangulation 101

A DNA triangle has three DNA matches who are all connected to each other genetically. Think of the length of each line in this genetic triangle as your relationship to one another, as measured by your DNA testing site in centimorgans.

 When most effective, your DNA triangle has two known points with a fairly short distance between them. Let’s say these two points are yourself and your second cousin Stan, who also descends from your great-grandparents, Jan and Ana Kaczynski. I like to call Stan our Best Match. The third point on your DNA triangle is anyone who is sharing DNA with both of you. Sometimes I call this person the New Match.

Find a New Match to triangulate by going to your testing site and looking for the tool that helps you identify matches you share with someone else (Stan).

  • At 23andMe, find the Relatives in Common tool under a header on Stan’s personal profile page.

  • At AncestryDNA, go to Stan’s profile page and clicking Shared Matches.

  • At Family Tree DNA, in your list of matches, check the box next to Stan’s name and choose In Common With from the top of the match list.

  • At MyHeritage DNA, go to Stan’s profile page and scroll down to see a list of shared matches.

Using this tool produces a list of other people who share DNA match with both you and Stan. All of these people are New Matches, forming their own triangle with you and Stan. So if there are 10 people on the shared matches list, you will have 10 triangles. Because these 10 people are all sharing DNA with you and Stan, you can You can theorize that they are all related somehow to Jan and Ana, either as their descendants, or as descendants of either Jan or Anna’s ancestors.

Using DNA triangulation to identify an unknown ancestor

What if you wanted to use triangulation to ask a different question, such as “Who are the parents of my ancestor, Richmond Claunch?” This is a question I asked myself. Richmond is my third great-grandfather on my father’s side, born in Texas in 1860. I had absolutely no clue who his parents were.

I’m fortunate to have my dad’s DNA, so I used his results as the first point in my triangle. I went looking for a second point on the triangle. It needed to be someone who connected with my dad on his Claunch side. Ideally, I want a match who descends from another of Richmond’s children (not the one through whom my dad descends). I didn’t have a known match who fit that description. So I searched my autosomal DNA matches on AncestryDNA for matches who had the Claunch surname, and then explored their trees until I found one (RS) who descends from Richmond’s daughter, Bessie. But if I hadn’t found another descendant of Richmond, I may have settled for a match who had Claunches in Texas.

Now I had two points of the triangle: my dad and RS. I needed the third. So I used the shared matches tool to find 7 New Matches shared by my dad and RS. These all form their own triangles, as shown in the image below under Round 1 (identified as #1-#7). Remember, each of these 7 should have some kind of relationship to Richmond and his wife Lilly as their descendants, or Richmond and Lily through their individual ancestors.

Shared Matches Blog.png

Round 2 of finding New Matches involves taking match #1, and running the shared matches tool again. Doing that finds two more matches who shared DNA with both my dad and match #1, but didn’t originally show up because they don’t share DNA with RS. Those two matches became matches #8 and #9 (under Round 2 in the image on the right). You can repeat this process for all of your New Matches, but for the sake of time and space, we are going to move forward with just these 9.

These 9 matches should fall into three groups:

  1. Those who are descendants of Richmond and Lilly, like my dad and R.S.

  2. Those who are descendants of Lilly’s parents or grandparents or great grands etc.

  3. Those who are descendants of Richmond’s parents or grandparents or great grands etc.

Exploring shared matches

We want to identify the third group of people if we hope to do research on Richmond’s line.

Even in the shared matches list between my dad and Match #1, we can see something interesting. Match#1 does NOT match #5 or #7. This likely means that #5 and #7 represent either Richmond’s line OR Lilly’s line. This theory gains strength when I realize that #5 and #7 both have Lilly’s maiden surname in their trees. I can then turn to genealogy research to confirm the theory that my dad, #5 and #7 all share ancestry on Lilly’s side.

But I won’t do that right now because I am laser focused on Richmond’s line and I’ve now identified some matches (#1, #2-4, #6, #8-9), who are likely just related to the Claunch side. These become my new priority matches to answer the question, “Who is Richmond’s father?”

Next up: it’s time to do genealogy research. I look at the online trees of these matches and try to figure out how they’re related to each other through traditional research methods. If I can find their common ancestor, I know that I am somehow related to that family as well.


Update fall 2019: I’ve just published a new step-by-step guide to help you try this yourself! Finding An Ancestor Using Your DNA leads you through the process of isolating your DNA matches to those specific to a certain ancestral line, and then exploring them for answers. The methodology I lay out in this guide is more simple and direct than what’s above, and it’s all about the steps YOU will take to apply these techniques. Best new (and affordable) DNA product out there!


Not a do-it-yourselfer when it comes to DNA? We can help you if you get stuck. We offer both mentoring sessions to help YOU do the work and expert research packages that leave the complicated genetics part to us.