DNA Triangulation

Diahan Southard

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DNA triangulation can help you figure out how you’re related to your DNA matches or identify an unknown ancestor on your family tree. Here’s how it can help you with your shared matches and genealogy.

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

Triangulation DNA diagram (1).pngA 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 theorize that they are all related somehow to Jan and Ana, either as their descendants, or as descendants of either Jan or Ana’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. (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 Lilly 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.

Now you try it!

You can DO this DNA thing, and we can help you. Start with our inexpensive Finding an Ancestor Using Your DNA quick reference guide, which walks you step-by-step through the process described in the article above. You’ll be working through your connection with matches and making new discoveries in no time!

I'd like more info on that Quick Guide!

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<a href="https://www.yourdnaguide.com/author/guideyourdnaguide-com" target="_self">Diahan Southard</a>

Diahan Southard

As founder and CEO of Your DNA Guide, Diahan Southard has been teaching people how to find family history answers in their DNA for several years, and she's been in the genetic genealogy field since its infancy. Diahan teaches internationally, writes for popular magazines, consults with leading testing companies, is author of Your DNA Guide–The Book, and producer of Your DNA Guide–the Academy, an online learning experience.


  1. Diane Gould Hall

    This is an awesome explanation of a tool I’ve avoided for lack of understanding. I have two long-standing brick walls. One is a great grandfather and I can surely solve that one because I have my Mom’s DNA and she is his granddaughter. The other is a bit tougher.
    Thanks so much Diahan.

  2. Diahan Southard

    Diane! I am so glad this was helpful. I can’t wait to hear what you find!

  3. Bob

    Word of caution… a shared match could be related to you (or others) in more than one way, thus complicating the determination of related group(s). Especially watch out for cases where you have multiple branches tracing back to the same geographical area in the same time period… or when working with shared matches who have incomplete trees or that also reflect multiple branches in the same areas.

    • Diahan Southard

      Great points, Bob! Thanks for clarifying.

  4. Carolyn

    I’ve tried this twice before finding this blog, and I’m completely baffled. I’m trying to find the families of 2 3rd great grandmothers, one with a maiden name, one maiden name unknown. In my recent research I’ve been looking at the 3rd match shared by myself and just one other person from a different branch, and I’ve been building the trees. Of 11, so far, 3 trace back to my known ancestor, but the other 8 have no common ancestors at all. None come from the same county as my known ancestor or migrated to the same locations as my 3rd great grandparents.

    In my other case I have about 45 unrelated trees; only 9 have a common ancestor between 2 trees and were not closely related, and there are about 7 have completely different sets of common ancestors. In this case I have a maiden name; none of the trees I built have that surname or any variation of it.

    I looked at 3rd party matches of several people with one common relative of mine, from different branches. How can these 3rd parties have so many different sets of ancestors yet be DNA related to me? This is all on Ancestry, so no chromosome browser is available. There are no known intermarriages of different branches.


    • Diahan Southard

      I think this is something that I would need to look at with you to try to see what exactly is going on. This would be a good topic for a mentoring session. Likely there are explanations for what you see, but I would need to get in and look to know what they are exactly.

  5. Tanya

    I’m trying to find the identity of the father of my 2x-great-grandmother, Ruth. There’s a crazy family story about how she came to be (the son of an English lord raped her mother Eliza–who was said to be "slow"–when Eliza was working for them as a live-in servant), and I’m curious about how much of it is true, if any. I’m just really mixed up and I don’t understand it well enough to know what I’m doing!

    But in looking into it, I’m also discovering I have a ridiculous number of relatives in the Eastern US, and I have no idea where they came from. I’ve noticed this before, that I had so many relatives in that area. But these seem to come from Ruth’s and her husband John’s line specifically.

    On top of that, I’ve discovered a relative in my first column who matches my 2x ggma’s maternal grandfather’s line, but that would make him my Dad’s 5th cousin (I’m using my Dad’s DNA), and DNA Painter says the chances of him being that distant are basically 0%. So I think we must connect to him somewhere else as well, but I don’t know where.

    And someone else in that column descends from an entirely different branch of my tree. Like, John and Ruth had my ggpa, also named John, who married Harriet. The DNA match I’m referring to in this paragraph matches Harriet’s line WAY back.

    • Diahan Southard

      Hey Tanya,
      You are right, figuring out which matches go to which line can be very complicated. If you are at AncestryDNA, you can use the Dot System to help you keep it all straight. I have a video up on You Tube that might help: https://www.youtube.com/watch?v=MRrEEDziYAs&t=31s.

  6. EDD

    I ended up with two mystery 1st cousins on Ancestry, one has 886 cms 43 segments, the other has 660 cms 28 segments. I cant find a connection except the cousin with 886cms has a maternal great grandfather with the same surname as my maternal great grandmother’s maiden name.

    On 23andme 886cms Ancestry cousin’s brother shows 771cms with me, their paternal uncle shows 1106cms with me and his two children show 722cms and 587cms.

    The 2nd mystery 1st cousin with 660cms has no family tree and has not replied to my messages. The other cousins do not know her.

    I dont know the identity of my father and for personal reasons, I cant say why. At this level of cms with the mystery cousins, could the possibility be they are from my father’s side. The one on Ancestry shows my children are her 2nd cousins. My mom and half brother only show as distant cousins with her.

    What help do I need to further research or is it a matter of continuing to research to discovery a family secret to make a connection?

    • Diahan Southard

      Matches who are not sharing with your mother, except distantly, are related to your father. So undoubtably, these matches are related to your father. You can use the data at the shared cM project to help you see your relationships (https://dnapainter.com/tools/sharedcmv4). We could go over it together in a mentoring session (https://www.yourdnaguide.com/thementor). In general, you start with the match who is most closely related and work out from there. So the match who is sharing 1106. I hesitate to draw any conclusions here, but it is possible that this match is your half uncle. If you have further questions, please use the contact form on our website here to email us.

  7. Brian

    I am going through triangulation son MyHeritage and have a quick query. Is there any correlation between the chromosome number on which a triangulation occurs and a particular ancestral line? For instance, if the triangulation for two matches is on chromosome 6 and connect through my 3x ggmother, and two other matches triangulate on the same line, is this significant or purely coincidental, in other words might they also descend from the same ancestral line?

    • Diahan Southard

      You have a good instinct here, but unfortunately, it isn’t always helpful. Lots of people are using triangulation in just the way you are suggesting. However, statistically speaking, it is highly unlikely, if not impossible that four individuals who are 4th cousins (or more distant) will share exactly the same piece of DNA handed down from the same common ancestor. So more likely is that it is just a common piece of DNA, representing a shared population. Perhaps you are all Irish, for example.

  8. Courtney Tolbert

    I suggested the colour doting system to Ancestry and was thrilled when it was implemented. I have been sorting my DNA results ever since, but am just as frustrated because I have not been able to utilize the information to narrow down several NPE’s in our bloodline. I have used Myheritage’s tools as well and have generated multiple reports. I feel that the information is just before me, but I can’t see it. I was fortunate to have my mother’s DNA; my father died in 1979, but his sister, my aunt and her son have their test done as well. I want to verify the parentage of my maternal 3rd great grandfather.

    • Diahan

      I agree, I love the dot system. Have you tried my book to help you? The dot system is best used to help you find your Best Mystery Matches. You may also consider just going over your research with a friend who understands this as well. Sometimes we are just too close to our own problems and can’t see the solutions right in front of us.


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