I think something about endogamy should be added to the Triangulation Help Page.
My uncle matches Alice and Bob (names changed for anonymity) on the same segment of chromosome 13, and MyHeritage reports that it triangulates.
My uncles grandmother was Josephine, and her parents were second cousins. So Josephine descended from Isaac Johnson and Mary Elwell via two separate lines, one through their daughter Harriet and one through their son Isaac. Alice descends from them through their daughter Mary Johnson. Bob connects one generation higher -- they descend from Mary Elwell's parents through their daughter Sarah Elwell.
So I have triangulation, and I have the three-legged school as written (even though several of us believe that requirement as currently written is overly restrictive). But there is endogamy, so I should not confirm any of my line from Josephine up to Isaac Johnson and Mary Elwell.
I suggest that the triangulation help page be modified to add clarity to what is already there, for instance, by using language that parallels that on the main DNA confirmation page, and to include Endogamy.
The current language is:
Requirements for Triangulation Simple one-to-one DNA confirmation is sufficient if your match is a third cousin or closer. Confirming relationships using more distant cousin matches requires triangulation. Three or more cousins need to all match each other on a single segment of DNA that is at least 7cM long. Seven cM is a bare minimum and presumes that the cousins' relationship back to their most recent common ancestor(s) is well-documented.
My Suggested Replacement
Requirements to Mark Relationships as Confirmed with Triangulation Simple one-to-one DNA confirmation is sufficient if your match is a third cousin or closer. Confirming relationships using more distant cousin matches requires triangulation. Triangulation is a process of comparison between yourself and two separate DNA matches. You cannot complete this process with just a single match between yourself and one distant cousin.
If your two matches meet the following criteria, you can mark each relationship back to your most recent common ancestor(s) as Confirmed with DNA. If your matches have Wikitree profiles, you can also mark as confirmed each relationship in their lines back to your most recent common ancestors.
your lines with your matches are well-documented and meet in a three-legged stool (described in detail below). This criterion is solely about your family trees — DNA does not get involved in this step of the process. you must perform a search for endogamy (described in detail below). This means you must do a thorough examination of your tree and your two matches' trees to look for multiple sets of common ancestors. Once again, DNA is not involved in this step of the process. you and your matches share a triangulated segment of DNA. Determining if you share a triangulated segment requires that you and both of your matches all have DNA tests at the same DNA matching service, and it cannot be done at Ancestry DNA at present. Instructions for triangulation at 23andme, MyHeritage, FTDNA, and GEDmatch are given below. Then below the section on Three-Legged Stool, I suggest a new section: Search for Endogamy
Before confirming with triangulation, a thorough search for endogamy must be performed. As mentioned above, you will need two DNA matches to begin the confirmation process. At this step, you must conduct a thorough search of your own tree and your two matches' trees looking for additional common ancestors beyond the ones you have already identified, and additional ancestral lines back from yourself to the common ancestors you have identified If you discover that you have a second set of common ancestors with one of the matches, then you cannot complete the triangulation process with these matches.