In the Chromosome Browser — One-to-many, some DNA segments are indicated by a frame. Those are the triangulated segments found for all the DNA Matches you selected.
Triangulated segments are segments that all of the selected DNA Matches share with each other. This capability is important for understanding DNA Matches’ relationships because triangulated segments are more likely to be inherited from a common ancestor.
Segments that seem to be shared by three or more people are not necessarily triangulated. For example, if you compare yourself with two people, call them A and B, you may see that you share a segment with A and you share a segment at the same genomic position with B, but A and B may not match each other on that segment. That’s because you may be sharing your maternal segment with A and your paternal segment with B and A and B aren’t related to each other at all.
If a match is shown as triangulated, it means that you, A, and B, all match each other precisely on that segment, and therefore all of you are probably related, and you probably got that triangulated segment from the same common ancestor.
Validating triangulation is complex, especially when comparing the DNA of a large group of people, and MyHeritage is the first company that does it automatically for you behind the scenes, for free.