All major genetic genealogy services show long lists of people who match part of your DNA. The problem is they don't show how you match. Is the match with your mother's or father's side of the family? Could it be from the Smith branch, or the Jones, or some other surname further back in your tree?
The only clue provided by the DNA testing company may be a "predicted relationship" based on the length of shared DNA. The shorter the segment, the more distant the relationship.
Let's say you have a DNA match with a predicted fourth cousin, Jennifer. The shared ancestor is likely a great-great-great grandparent. While you know Jennifer's full name, her last name is not on your tree. Even if you knew all 32 of your great-great-great grandparents' full names, Jennifer may not know much about her own tree going back that far, making it difficult to narrow down how you are related.
However, if you are able to identify a third person who matches both you and Jennifer on the same DNA segments, then it's possible to learn how you are all related.
For instance, if you and your great-uncle Clarence share the same DNA segments with Jennifer, that lets you quickly determine which branch of the family she comes from. Instead of wondering which one of your 32 great-great-great grandparents is the shared ancestor, you can narrow it down to the eight great-great-great grandparents in the same branch as your great-uncle Clarence.
You and Jennifer can then start to compare notes about surnames, places of birth, and other details that can help you identify the shared ancestor. Jennifer may even have information about that ancestor that you don't, which can help you strengthen your own tree.
In short, benefits of triangulation include:
- Identifying ancestors
- Confirming ancestors
- Connecting with relatives
- Determining how distant cousins are related to you
- Filling out other branches of your tree
- Discovering new surnames on your tree
- Learning more about the origins of specific ancestors