A family therapist has to know what a triangle in family system, is how it has developed, the process of the triangle, the effect of the triangle on the family members, whether it`s effects are negative, and how the family members have been effected. Because family therapist cannot understand the family without knowing the about. Everything in the world is apt to keep an homeostasis. If there is no security between objects, they need another part in order to have homeostasis. This principle is same in family system. If there is no homeostasis in marrital system, the third person invades into the married couple. The triangle happens when the family members come in and out of the family. It is necessary to have a knowledge about the triangle to see the problems of family system.