Triangulation: Proving a Common Ancestor

The same question seems to come up over and over again among those new to autosomal DNA testing. If I match A and B on the same segment why is that not enough to prove they match each other and we have a common ancestor?

The reason the ancestor is not proven is that you have two strands of DNA on each chromosome (remember there are 23 pairs of chromosomes) and the testing mechanism cannot differentiate between the two of them. So A could match the piece from your mother and B could match the piece from your father or one of them could even be a false match to a mix of alleles from both parents (see my post on IBC for more on that concept)

The way to prove the common ancestor is to see if A and B match each other in the same place that they match you. This is what we call triangulation.

Triangulation example

Kristine’s shared DNA with other Wold descendants, relationships are to her (to me in parenthesis)

About a year ago I blogged about how, after many years, a change in spelling on the paper trail had led fellow genealogist Dennis to think his wife Kristine was perhaps descended from my great-grandmother’s brother Carl. To prove this I suggested he test her autosomal DNA.

I also convinced another Wold cousin to test in order to help out. This was a good thing since Kristine shared no DNA with my nuclear family but she did share DNA with my first cousins (brothers Henry and George), my 2nd cousin (John), and much DNA with the tested Wold cousin Mike (my third once removed).

Every place you see two parallel colored lines in the diagram above, two of my Wold side cousins are matching Kristine’s DNA and each other in the same spot. That is triangulation. Because her matching DNA triangulated with several different cousins in three spots, we are now confident that Kristine shares our gg-grandparents Anna and Jørgen Oleson Wold (click here for the full story). [addendum 19 Oct 2015: this diagram was updated to the one used in my triangulation presentation which includes cousin, Katy, found via DNA testing after this article was originally posted. She is a 2nd cousin 1R to Michael. The details of this triangulation are shown in my triangulation presentation]

Looking through the master spreadsheet for my Dad, I see a reverse case. Jeff matches my Dad and me from 46M to 57M on chromosome 2 for about 10cM while LuAnn matches my Dad and my brother from 46M to 58M also on chromosome 2. Even though they each match Dad in the same spot, they do not match each other there. Plus one matches me and not my brother while the other matches my brother and not me. This is a good example of a match that is proven to be from different sides, one maternal and one paternal, and thus different ancestors. In other words, a pair of matches at the same spot that did not triangulate with each other, but did triangulate with other family members.

So how do you do a triangulation?

The easiest place to do a triangulation is – there you can use the one-to-one comparison tool to compare A to B, B to you, and A to you and then see if all three share the same segment(s).

On 23andme it is also easy, provided you are sharing with both A and B. You use the Family Inheritance Advanced tool (under Ancestry Tools on Your Results) to compare all three. When you see that A and B match you, then you switch the person on the left to A or B and put yourself on the right with the other one. Now you can see if A and B match each other at that same spot. Below is the example with Jeff and LuAnn from the case previously mentioned. In the first one LuAnn is compared to my family and Jeff. Then I switched her and Jeff for the next compare.




On Family Tree DNA it is more complicated to triangulate since you cannot directly compare A to B, you can only compare them to yourself or any other kits you manage. One possibility is that when both A and B match you on a specific segment you could contact each of them and ask them to check if they match the other at that location. A workaround that many use, however, is the In Common With (ICW) function. As in the example below, you can click below a person listed in your family finder match list and request to see all the people the two of you match in common by clicking Common Matches and then In Common With.


If A and B are in common with each other then it is likely that the segment where they both match you is a match. But it is no guarantee. The number I have heard is 90% but that is just an estimate based on experience; others say 99%. Below is a case where both Gunnar and Stephen match my Dad and they are each on each other’s in common with list. So a likely match.


Is there anything better? Well you can use other family members to check the match.

Since both my brother and I have tests at Family Tree DNA as well, I used the surname search to find these kits in my brother’s match list, then viewed them in the chromosome browser. If these two guys are a match, my brother will either match them both or none. Why you ask? Most likely, my brother only has either Dad’s paternal segment or maternal segment at this spot. DNA tends to travel in chunks and although it may get smaller as it gets passed down, it takes many generations for it to get more mixed up. Here is my brother’s view of these two in the chromosome browser:


By the way, neither of these fellows matched me. Have you noticed that this is the same spot on chromosome 2 that we were looking at for LuAnn and Jeff? So what else might we learn from this?

Well remember that I matched Jeff and my brother matched LuAnn so we each got different segments from our Dad here. Thus LuAnn must match Gunnar and Stephen as well. I am hoping Stephen and I will find this common ancestor for our 16.9 cM match, but no luck yet, and I have not heard back from the other two, a common problem …


Further reading on triangulation:

This blog post of Roberta Estes:

This article at the ISOGG wiki:

22 thoughts on “Triangulation: Proving a Common Ancestor

  1. This is very helpful. Is there a way to use the triangulation tool on GEDmatch rather than the one to one tool to compare results of five or six people? I have tried sorting out my results using FTDNA, DNAgedcom’s ADSA, and GEDmatch, and I always just end up confused and frustrated. I am just trying to compare results of my mother, brother, a known second cousin, and three or four close (2-4th cousins) matches about whom I do not know the connection. I just keep getting lost!

  2. There is also a matrix tool at FTDNA that can be used to compare multiple matches to each other. The only thing here is that you can’t control the threshold for cm. It’s just a yes or no colored box in the matrix but still helpful nonetheless.

    Another useful tool is Genome Mate.

  3. The best I have found for this is a “Tier 1” tool (make a $10 donation to their efforts for a month of access) on gedmatch. Enter any gedmatch number and you will find a listing with each line containing 2 people that triangulate (each kit#, name & email), and the common segment for the 3 of you (chr, start, end & length).

  4. Here’s my situation:

    Family tradition says that Diana, Barbara, and Rose share a common ancestor.
    Diana & Barbara match on chromosomes 1,4,6,9,10, 13-17, 20, & 22.
    Diana & Rose match on chromosome 9
    Barbara & Rose match on chromosomes 3, 4, 7, 10, & 16.

    So, Rose is related to both Diana and Barbara, but since no chromosomes are shared in common among all three, does that mean Rose is related to Diana via a different ancestor than she shares with Barbara?

  5. Triangulation proves a common ancestor but the lack of triangulation does not prove that there is a different ancestor. When LuAnn and Jeff did not triangulate in my post, there was triangulation with other members of my family to get to the conclusion of different ancestors.

    So your case does not prove that Rose is related via a different ancestor. You need more data (more cousin tests) to prove or disprove that theory.

    • Well, I do have two more cousins involved.

      Diana & Sylvia match on chromosomes 4, 6, & 9.
      Barbara & Sylvia match on chromosomes 2, 6, & 20.
      Rose & Sylvia do not match.

      Doug & Diana do not match.
      Doug & Barbara match on chromosomes 1, 6, 20, & 21.
      Doug & Sylvia match on chromosomes 1-3, 5-6, 11, 16-18, & 20.
      Doug & Rose match on chromosome 13.

      One potential problem is that Doug, Diana, Barbara, & Sylvia’s trees have more than one ancestor in common.

      I am feeling pretty lost and don’t know what to make of all of this. Thanks for your input.

    • Terry I took a quick look and nothing jumps out at me. Get more people tested. I will comment over at facebook.

      And I do apologize, I cannot afford to spend much time on individual cases that are not my relatives or clients. But I do enjoy helping as I learn from every experience.

  6. Having more than one common ancestor makes it more difficult and more complicated. The only time you can be confident there is a common DNA ancestor is when 3 people match in the same spot.

    Do Doug, Sylvia and Barbara all match in the same place on chromosome 6? Do Diana, Sylvia, and Barbara match on the same spot on chr 6? If not then you cannot conclude anything from what you are seeing yet. Get more cousins tested. Getting the oldest generation to test can be very useful.

    What is your objective with this testing?

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  9. I have read your blog on autosomal dna and am still having difficulty identifying ancestors. I have tested5 family members and have 2 additional kits for my sister and my aunt. I have to say my dna is confusing particularly since an ancestor in my mother’s line appears to have married an ancestor in my father’s line. I thought this was unlikely since my mother’s line which matches appears to be irish and my father’s line is strictly polish. I have found however a story by several of my dna matches where a polish ancestor of their’s fled Poland by swimming a river and then married an irish girl. It seems likely by triangulation that the irish girl is a descendant of one of my irish lines however it has been extremely difficult to link lines with irish records. I am in general having difficult identifying chromosome levels with ancestors as many of my links where I know a MRCA are back in generation 7 or beyond with my cM match <5 although we may match on 5 or 6 different chromosome levels Is it possible to identify chromosome matches at a smaller level mostly if you also have a larger match with another individual. If you have identified an individual through a larger match, does it then stand to reason that the smaller match must also be that surname. I have started using the X-chromosome more and hope that I can identify individuals easier. Helen

  10. I have a basic concept question about autosomal testing and chromosome segment matching. Can the segments of each chromosome be mapped as unique to either the maternal or paternal side, or are all these segments mixed up and you need to still determine which side someone comes from by comparing trees and triangulating with ancestors from each side? An example, when I look at myself, my mother and my aunt (my aunt is my mother’s half sister-they share the maternal line). The segments we all share in common on each chromosome must come from my maternal grandmother’s ancestors- the line we all share. If I look at any others who share parts of the segments we three share- then can I assume they are through my grandmother’s ancestors? 11% of my DNA matches my aunt, so that 11% we share through my grandmother’s ancestors. Does this mean that anyone else who matches us in those segments must also come from my grandmother’s side? An answer to this would help me better understand the whole idea.

  11. You need to always use your matches to others to map out your chromosome. As I say in the article above “the testing mechanism cannot differentiate between the two” chromosomes, paternal and maternal.

    Having a parent tested does let you see which side a match is from. And a half aunt is very helpful. Yes, your common matches with her must come from your maternal grandmother’s ancestors unless it is a false match for them which can happen with smaller segments. See my post at for an explanation of false matches

  12. Through 23andme I learned I carry the Cystic Fibrosis CFTR gene mutation F508. This is on Chromosome 7 in the 117,xxx,xxx region. With ADSA I found I have a triangulated group in this region. My question is this: in the absence of informative ancestral trees, would the chance that any one member of the triangulated group also carries the mutation be 50%? Since there is no way of knowing if I or any of them have it on the maternal or paternal chromosome, right?

    • Melinda,
      You are correct, there is no way to tell from just your test and I presume neither of your parents is tested to isolate the side that way.
      But if you have other cousins tested who match you on that segment then you may be able to infer which side the CFTR is on from the cousin’s test. Then if you can compare your matches to your cousin (or have her send a screenshot of her ADSA results at that position) you can tell which ones may have it as well

  13. I got a question. I have a relative on gedmatch who apparently shares enough autosomal DNA to be considered not just a cousin but a full sibling. Unless my parents adopted out some unknown sibling of mine, I possess one full sibling & one female half sibling [the relative is male apparently]. 3523 versus the 2678 I get with my sibling, 1.0 generations versus 1.2 generations.

    Thing is, is this relative doesn’t share any chromosomes with myself or my sibling. Nor for that matter grabbing a bunch of random relatives and using gedmatch’s chromosome segment matrix anyone else.

    So is he a relative or some glitch?

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