Your have one-to-one matches but it is possible that you both match a third party in a different place on the chromosomes. You may match on chr5 for person A and chr 11 for person B but A and B also have a match between them. This proves you are all related but not to a common ancestor. You each have a common ancestor in different areas of your own tree. Between the three of you, there is a better chance of finding the area of investigation for a common ancestor.
This is useful if -
You do not have a DNA match with someone but do have other evidence to believe there is a link between you.
You should have a DNA match based on your trees or other evidence but do not get one.
What you need then is someone who has a DNA match to each of you - an intermediary of sorts? Note this match is not the same chromosome or the same segment of any chromosome.
GedMatch does this for you as do other sites. Put your kit number and the other persons DNA into the "People Who Match both of 1 or 2 Kits Tool" and it will generate a list of any third party DNA matching both of you and the other person from the ten million plus members.
Please note that It will also give you contact details to enable you to write and introduce yourself. BUT before you do that, you MUST take each kit number of interest and run a one-to-one test to make sure that the result is true!
Now this is often confused withJoint Matches. In Triangulation the match with 2 or more people is on the same region of the same chromosome. It is much more specific. The segments do not have to be exactly the same length but ideally be very similar overlaps as in the example below.
The significance of this is that all 3 of you have the same common
ancestor line - for example you may relate to the father of the family and
two others may relate to different children.
This works in a very similar way to Triangulation but uses many of your best one-to-one DNA matches. It takes the list of the top 100 or 500 matches and tests them against each other. Where it finds all the people who triangulate at the same part of your DNA it will make a group called a cluster. Each cluster represents one area or branch of your tree and all the people in the cluster got their DNA from the same ancestor. However, that can be a lot of people to write to. Take the biggest group and start there. If your tree and research is advanced you may be able to spot a group that is more interesting to you and use them as your focus. You may note an ethnicity or a common surname of particular interest in their cluster or profiles?
Seamus Crowe Website2021