According to Murray Bowen, a three-person system, also known as a triangle,
is the molecule or the basic building block of any emotional system, whether it is in the family or any other group. The triangle is the smallest stable relationship system.
On the other hand, a two-person system "is unstable in that it forms itself into a three-person system or triangle under stress." When tension mounts, a two-person system will automatically triangle in another person.
Phillip Klever argued that all marriages begin as triangles because everyone has "some degree of unresolved attachment" to their parents, or "the inability to be a separate individual while being involved with one's parents". As often is the case, the adult child's spouse becomes involved in dealing with the unresolved attachment between the adult child and parent. Also, the parent is vulnerable to being triangled in when the marriage becomes too emotionally intense for both spouses.
In the case of covert incest, a marital triangle often consists of an adult child (e.g., husband), the adult child's spouse (e.g., husband's wife), and the adult child's parent (e.g., husband's mother).