A Triangulation Overview

A Segment-ology TIDBIT

Triangulation is a tool. It’s a process that can help us with our genealogy. It is not the only tool in our kit bag – there are many other tools that also utilize DNA, including InCommonWith Lists, Matching Segment Lists, Matrix displays, Shared Matches, Clustering, Circles, etc, etc. This blogpost is an overview of Triangulation.

With atDNA we have been using Triangulation to mean two different things:

Segment Triangulation of shared segments (a focus of this blog), and

Ancestry Triangulation (having at least 3 Matches in a Triangulated Group (TG) all match on the same ancestral line; sharing a Common Ancestor (CA) on that line.

In the atDNA community we often conflate these two concepts, and they are very much intertwined. I tend to think first of forming a TG and then looking at the genealogy to determine the side (maternal or paternal) and then finding various MRCAs. But some start with the genealogy and look for Triangulation to add evidence that a CA is correct. Both ways will work, they are intertwined in genetic genealogy, so in this overview I will also conflate them. Here are some overview points about Triangulation:

We look for at least 3 Matches, Much of our work as genealogists involves one-on-one – finding a Common Ancestor with a Match – that’s OK, but it’s not Triangulation.

We look at overlapping DNA segments. ICW and other tools don’t require overlapping segments – that’s OK, but they are not Triangulation.

We look for 3 “segment” legs. This means the 3 people (usually you and two Matches) that form a Triangulated Group are not closely related. But once a TG of 3 cousins is formed, other close relatives can be added to the TG. It’s the TG forming that needs 3 strong legs. So 3 siblings and their parent do not form a TG, but they can be in one.

The shared segments that form a TG must be IBD. From experience we’ve found that:

  • “all” shared segments over 15cM are IBD;
  • shared segments under about 7cM are false most of the time; and
  • the process of comparing overlapping shared segments in a TG will cull out many in the 7 to 15cM range which do not match – I consider these to be false segments.

Blaine Bettinger is working to define Triangulation – not to preclude the use of other tools – but to help us better understand Triangulation as a tool. I use Triangulation as a tool to primarily sort and group all of my IBD segments. I’ve formed about 400 separate TGs over my 45 chromosomes. New Matches always fall into one of these TGs (close Matches may span two or more TGs – it’s OK). This is segment Triangulation. With close relatives, I’ve been able to determine the side for these 400 TGs. This is a huge benefit because new Matches almost always Triangulate with other Matches already in a TG; and I then know which side our Common Ancestor must be on. This is an excellent use of the Triangulation tool.

Ancestry Triangulation does not preclude me from also using the information of Circles, or ICW lists, or ethnic makeup, of even genealogy records or Trees or discussions with Matches to determine CAs.

Within a TG we may find a CA with a Match. As we have pointed out many times: shared DNA plus a CA does NOT mean that the shared DNA came from that CA, or that the CA is somehow “proved” because there is also a shared segment – maybe, but also maybe not. But, by finding 3 Matches in a TG who all share the same CA (Ancestry Triangulation), we increase our confidence (not “prove”) that the CA is linked to the shared segment; and with more Ancestry Triangulation (and/or walking the ancestor back), we increase our confidence even more.

If Triangulation leads to a conclusion that a CA is not linked by the shared DNA, we can still be cousins on that CA, and we can still use ICW, Circles, etc. to pursue a genealogy goal. But we should not say that DNA supports that cousinship conclusion.

IMO, a TG has characteristics that help us in our genealogy goals. Triangulation is a strong tool that takes advantage of our shared DNA with Matches.

I applaud Blaine’s effort to try to define Triangulation and provide some standards for its use.

The above is adapted from my recent post to the Genetic Genealogy Tips and Techniques Facebook Group.


[22N] Segment-ology: A Triangulation Overview TIDBIT; by Jim Bartlett 20170728

12 thoughts on “A Triangulation Overview

  1. So, I have a group of 40 matches all overlapping and sharing with each other averaging 12cM, range 10cM to 42cM, with only 4 matches above 15cM would you be discounting the other 36? Or, is the fact that so many share with each other of significance?


    • I’d keep them all. You never know – a 12cM Match may be a 4C with lots of research done on the CA; and the 20cM an adoptee, or no Tree, or not interested in genealogy, or 2C to someone else who paid for the kit and is not interested in helping any Match on other lines, or dead; or…. You never know where the gold is until you prospect for it.


  2. Great article. It helped clear up a few things for me. Regarding the 3 “segment” legs, are first cousins too close to make separate segment legs? You mentioned that parent and siblings would be considered one leg. It would seem that it might be counterproductive (or at least not of much use) were I to use first cousins on separate legs from that (assuming that the first cousins relate to everyone else on that leg and that they contain the segment I am working with, e.g., it’s the same MRCAs).


    • Elizabeth, you can use 1C. The idea is to form the TGs, and have them on both sides. I still “adjust” my TGs as more data (segments) come in. These are guidelines. And I’d add that it isn’t a requirement to only use known cousins – you can use any overlapping segments to form and fill up and fill out a TG. Most of your TGs will probably have more than just 3 segments.

      Liked by 2 people

      • Let me add that in using Triangulation for your own purposes, you had hedge on these guidelines – they are intended to significantly improve your odds. If you do hedge, you may have to backtrack and/or change more often as new data comes in. When I started with atDNA, I just focused on finding Common Ancestors – I’m a genealogist, what can I say. After about 2 years I had a list of about 100 Matches with Common Ancestors and put them into my first spreadsheet. I then started adding segment data, and learning about atDNA. It turns out about 1/4 of the Common Ancestors could not be right – they were on the wrong side; or they were on the same segment with a close cousin, but a different CA. A lot of work wasted because I didn’t use the DNA. And the other thing is if you want to state that an ancestor is linked to a DNA segment, you need to follow the generally accepted “guidelines” to promote a high confidence level that you are correct. I predict future “disputes” over ancestry (like we have today with paper genealogy), and the one who has followed the guidelines should win.


  3. I certainly agree with you, Jim. On the issue of whether the term “triangulation” can be applied to autosomal comparisons involving fewer than three people… NO! Please let us reject this idea immediately. Although this conceptualization makes some sense with Y DNA comparisons (because the Y chromosome is not diploid), this conceptualization falls short with our diploid autosomal DNA. When working with autosomal DNA, we cannot rely on one-to-one comparisons to achieve what has been called “triangulation” with Y DNA.


  4. A note that’s a bit tangential: Although I’m sure the readers of this blog understand this very well, one point that often seems to be missing these conversations is any recognition of the fact that triangulation is — in and of itself — a consideration of the possibility of DNA inheritance through a line other than the hypothesized common ancestor(s). In other words, we triangulate because we recognize the possibility of alternative shared ancestors. Although triangulation doesn’t eliminate the need to search conscientiously for all shared ancestors — it does, to some degree, address that issue directly.


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