The "C" Factor

A and B - relationship (inside ones)
C - third person (outsider)

"In family systems theory, whenever two people have problems with each other, one or both will "triangle in" a third member. People respond to anxiety between each other by shifting the focus to a third person, triangulation. In a triangle, two are on the inside and one is on the outside."
From where





"The anxiety or tension of each of the participants generates the activity in triangles. A two-person relationship (A and B) will be stable as long as it is calm. When anxiety increases, however, the presence of a third person (C) can decrease anxiety by "spreading it out" over three relationships. This makes it less likely that one relationship will overheat. (...)  
 If you understand the position you occupy in a triangle, it may be possible to bring it to a positive conclusion."
From where