Triangulation with GEDmatch

On 25 Mar 2017, the Central Indiana DNA Interest Group is giving an in-depth presentation on the valuable (and mostly free!) DNA tools at GEDmatch.

At this program, we’ll be talking about the benefits of GEDmatch and walking through how to use the site. Of course, the most Basic perk of GEDmatch is that you can compare DNA from people who tested across different platforms (companies). On an Intermediate level, GEDmatch has tools to help you sort your matches into different lines of your family tree. For participants open to dipping their toes into Advanced territory, we’ll talk a bit about * triangulation *.

I know when I attend webinars or presentations, sometimes my brain gets full and doesn’t process everything I just heard. Then it’s helpful to have a resource to revisit later to help the more complex material sink in. So I thought I’d post something about * triangulation * on my blog this month!


Let’s say we have three DNA testers: Ann, Bob, and Mary.  See Image 1.

Image 1.


Bob Mary

They all share DNA, predicted to be 4th or 5th cousins. Ann matches both Bob and Mary. Bob and Mary also match each other. The only person who appears in both Bob’s tree and Mary’s tree is George Washington. Does this mean that Ann is descended from George Washington too?

Well… maybe.

Maybe not.

Suppose the DNA segments where they match are on the chromosomes shown in Image 2. (AncestryDNA doesn’t tell you on which chromosomes you match. FamilyTreeDNA will show Ann on which chromosomes she matches Bob and Mary, but only Bob or Mary can see where they match each other. 23andMe does show who matches whom where, but only if those participants all agreed to share, and of course, tested at the same company. If all three testers copy their raw DNA data to GEDmatch, they can see where they match each other.)

Image 2.

DNA matches w chrom

Now, just because they don’t match on the same segment does not mean they do *not* share the same common ancestor. Those three segments on different chromosomes might have all come from George. But they might not. More documentary research and more DNA matches might ultimately reveal that the DNA segment on chromosome 14 came from George, the DNA chunk on chromosome 1 came from Ben Franklin and the segment on chromosome 6 came from Thomas Jefferson, such as shown in Image 3. It’s too soon to tell. (Remember, Ann only knows a little over half her ancestors in that generation: she has no idea that Ben and Thomas are in her tree.)

Image 3.

ABM ancestors w chr

In this case, it’s entirely possible that Ann is not descended from George Washington at all. Each pair of distant cousins has a different common ancestor.

But what if they all matched on the same (or overlapping) segment on the same chromosome? If it’s a reliable size (say, over 10 cM), then that piece of DNA most likely did come down to each of them from the same person. They would all share the same common ancestor.

GEDmatch will show you if this is the case. See Image 4. The source person in that sample is Ann, who tested at FamilyTreeDNA. This screen capture shows Bob (line 1, who tested at AncestryDNA) and Mary (line 2, who tested at 23andMe) and where they each match Ann— on the same segment of chromosome 7.

Image 4.

triangulation sample

Now, Ann has two copies of chromosome 7: one from her dad and one from her mom. What if Bob matches Ann on her father’s side and Mary matches Ann on her mother’s side? Then they wouldn’t match each other there. So we also have to compare Bob to Mary, one-to-one.  We can do that on GEDmatch. And it turns out, they do match each other there too.

This means that Ann and Bob and Mary all have an identical chunk of DNA, passed down from the same common ancestor.

Was it George Washington? 



There are still two or three issues we have to consider.

Number 1. Each of the three testers still has blank lines in his/her pedigree charts. The common ancestor could be one of those unknowns. There are steps we can take to try to improve our confidence that we’ve picked the right ancestor. Looking at geography, migration, and history may help eliminate some lines in Ann’s tree to compare to Bob’s.

For example, Ann may not know almost half of her 4x-great-grandparents, but if she knows most of those missing names were still in Ireland before the Great Famine and if Bob’s missing names were most likely German Palatines, then those lines are not likely candidates for the common ancestor.

Number 2.  Since Ann and Bob and Mary have unknowns in their tree, it’s possible that more than one different person/couple may appear in all three trees. This is less likely the more recent the common ancestor and the more your trees are complete.

Number 3. Even if you use documentary research and DNA analysis to conclude that the common ancestor is in the George Washington line from Bob and Mary’s tree, Ann’s ancestor could be farther back. She could be descended from George’s brother Sam and the common ancestors would be George’s parents. Still, it’s valuable progress to have narrowed down which line everyone has in common. Right?

Triangulation gives us that. It’s a valuable tool.

It’s probably easiest to use if you start with two people whose common ancestor is very clear-cut: perhaps 2nd to 4th cousins, where you share the same brick wall. Then look for new cousins who triangulate with both of you, compare trees to find the common line and share new research leads.


In the spirit of St. Patrick’s Day, I’m definitely looking for some new-but-distant cousins with Irish roots to copy their DNA data to GEDmatch so we can confirm how we’re related. (Of my 16 great-great-grandparents, 8 are Irish: half on Mom’s side and half on Dad’s. I know at least five different counties they come from, but have no idea on the other three. So triangulation will be very helpful to me to figure out which branch of our trees we have in common.)

I’m particularly interested in Flynn descendents, Lawlor cousins, and Cahill/Connors kin. I’d love to find evidence of where in Ireland those lines came from, and I’ll be happy to share all my documentary research and DNA analysis with anyone who can help!  I’m A876964 on GEDmatch (and I’ve tested many more relatives you might match.) If you have questions you can reach me at


Ann Raymont (c) 2017







2 thoughts on “Triangulation with GEDmatch

  1. Pingback: An open letter to my DNA cousins | DNAsleuth

  2. Pingback: How reliable is our DNA evidence? | DNAsleuth

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