The ARC triangle is a central teaching of the Church of Scientology. Scientology is, at its core, about gaining understanding, particularly understanding of the self and how it relates to the rest of the world. The three components of the ARC triangle lead to greater understanding, while a lack of those components damages understanding.
CommunicationARC is an abbreviation for Affinity, Reality, and Communication. Communication is the most important of the three, and also the most straight forward. Without the exchange of information, there can be no increase in understanding, not formation of affinity and no understanding of another's reality. Because of this, the ARC triangle is drawn as an isosceles triangle, with the two longer sides pointing toward communication and the other two points of the triangle being affinity and reality.
AffinityAffinity is the emotional attachment between people. The goal is for people to have greater affinity for each other, to like and respect them rather than to be angered and frustrated by them. It is difficult for people angry at one another to communicate, and it is difficult to agree upon a reality due to vastly different perceptions about a subject.
RealityReality is agreement about what is going on in the physical world. Scientology stresses that our understanding of the world comes through our senses. Thus, our understanding of the world might not reflect what is actually going on. A blind person has a very different understanding of the world than a deaf person, for example. Each person has their own reality, their own way of understanding the world. For understanding to occur, two individuals must have some common grounds between their realities.
Realities at odd with each other need not be so dramatic and obvious. A couple arguing also has reality issues. A husband may see reality as a controlling wife trying to dictate his every move, while the wife sees reality as an uncaring husband who never seems to have time for family or who doesn't respect her time by coming and going without warning or consideration. There cannot be agreement until each side accepts some aspect of the other's perception of reality.
Unity of the ThreeAll three are dependent on one another. The decrease of one will lead to a decrease in the others. As such, it is important to address issues early so that one does not sink into a spiral of diminishing ARC.
To return to the example of the blind man and the deaf man: each has their own reality, one lacking visual input and the other lacking auditory input. Until they can find some common ground in how they experience reality, they cannot usefully communicate. After all, describing something as blue means absolutely nothing to someone who has been blind since birth. Without useful communication, these two individuals would become highly frustrated with each other.
But improving one point can improve the others. Communication might be improved by an interpreter using sign language to convey the blind man's words to the deaf man. And reality can be improved by focusing on things both people can understand: emotions, smells, tastes, textures, actions. Now they can have useful conversation and share ideas. They can form a positive bond, finding each other to be interesting and informative.