Bowen described triangles as the smallest stable relationship unit (Kerr and Bowen, 1988: 135).

The process of triangling is central to his theory.  (Some people use the term 'triangulation', deriving from Minuchin (1974: 102), but Bowen always spoke of 'triangling'.)

Triangling is said to occur when the inevitable anxiety in a dyad is relieved by involving a vulnerable third party who either takes sides or provides a detour for the anxiety (Lerner, 1988; James, 1989; Guerin, Fogarty, Fay and Kautto, 1996).

An example of this pattern would be when Person A in a marriage begins feeling uncomfortable with too much closeness to Person B.  S/he may begin withdrawing, perhaps to another activity such as work (the third point of the triangle).  Person B then pursues Person A, which results in increased withdrawal to the initial triangled-in person or activity.  Person B then feels neglected and seeks out an ally who will sympathise with his/her sense of exclusion.  This in turn leads to Person A feeling like the odd one out and moving anxiously closer to Person B.

Under stress, the triangling process feeds on itself and interlocking triangles are formed throughout the system.  This can spill over into the wider community, when family members find allies, or enemies to unite against, such as doctors, teachers and therapists.

Under calm conditions it is difficult to identify triangles but they emerge clearly under stress.

Triangles are linked closely with Bowen's concept of differentiation, in that the greater the degree of fusion in a relationship, the more heightened is the pull to preserve emotional stability by forming a triangle.  Bowen did not suggest that the process of triangling was necessarily dysfunctional, but the concept is a useful way of grasping the notion that the original tension gets acted out elsewhere.  Triangling can become problematic when a third party's involvement distracts the members of a dyad from resolving their relationship impasse.  If a third party is drawn in, the focus shifts to criticising or worrying about the new outsider, which in turn prevents the original complainants from resolving their tension.

According to Bowen, triangles tend to repeat themselves across generations.

When one member of a relationship triangle departs or dies, another person can be drawn into the same role (eg. 'villain', 'rescuer', 'victim', 'black sheep', 'martyr').  For example, in my own family of origin I found myself moving into the role of peacemaker after the death of my mother, who had mediated the tension between my father and brother.  This ongoing triangle served to detour the anxiety that had been played out between fathers and sons in the family over the generations.