This model is inspired by Bühler (1934) and Watzlawick et al. Bühler distinguishes "three aspects of language": representation (= factual content), expression (= self-revelation) and appeal. Watzlawick distinguishes between the content aspect and the relationship aspect of messages. The "content aspect" is equivalent to the "factual content" of the present model.

The "relationship aspect", on the other hand, is defined more broadly in his work and basically comprises all three: "self-revelation", "relationship" (in the narrower sense) and "appeal", and thus also the "meta-communicative" part of the message, which provides clues as to how it should be understood.

Watzlawick sees the advantage of this four-sides model in the fact that it allows a better classification of the variety of possible communication disorders and problems and opens the view for different training goals to improve communication skills. Watzlawick gives an example from everyday life: A man (= sender) says to his wife (= receiver) sitting at the wheel of a car: "Hey, there's a green light ahead!"
The four sides of this message are explained below:

Content

(or: What I am informing you about) Zunächst enthält die Nachricht eine Sachinformation. Im Beispiel erfahren wir etwas über den Zustand der Ampel – sie steht auf Grün. Immer wenn es „um die Sache“ geht, steht diese Seite der Nachricht im Vordergrund – oder sollte es zumindest. […]

Appeal

(or: What I want you to do)
Hardly anything is said "just for the sake of it" – almost all messages have the function of influencing the recipient. In our example, the appeal might be: "Put your foot down, and we'll still make it through the traffic lights!"

The message thus (also) serves to induce the recipient to do or refrain from doing certain things, to think or feel. This attempt to exert influence can be more or less overt or covert – in the latter case we speak of manipulation. The manipulating sender is not afraid to put the other three sides of the message at the service of the appeal. The reporting on the factual side is then one-sided and tendentious, the self-presentation is aimed at achieving a certain effect on the recipient (e.g. feelings of admiration or helpfulness); and the messages on the relationship side may also be determined by the secret aim of "keeping the other person happy" (e.g. by submissive behaviour or compliments). If the material, self-revelation and relationship sides are directed towards improving the impact of the appeal side, they become functionalised, i.e. they do not reflect what is, but become the means of achieving the goal. […]

Relationship

(or: What I think of you and how we feel about each other)
The message also shows how the sender feels about the receiver, what he thinks of them. [...] Generally speaking: Sending a message also always means expressing a certain kind of relationship to the person addressed. Strictly speaking, of course, this is a special part of self-revelation. However, we want to treat this aspect of the relationship as different from that, because the psychological situation of the recipient is different: on receiving the self-revelation, he is a diagnostician who is not himself affected ("What does your statement say about you?"), on receiving the relationship side of the message, he himself is "affected" (often in the double sense of the word).

Strictly speaking, there are two types of messages gathered on the relationship side of the message. The first type shows what the sender thinks of the receiver, how he sees him. In the example, the man indicates that he thinks his wife is in need of help. On the other hand, the relationship side also contains a message about how the sender sees the relationship between himself and the receiver ("this is how we relate to each other"). If someone asks another person: "Well, how is marriage?" then this factual question implicitly contains the relationship message: "We relate to each other in such a way that such (intimate) questions are quite possible.” Of course, it is possible that the recipient does not agree with this definition of the relationship, and considers the question to be out of place and intrusive. And so it is not uncommon for two interlocutors to engage in an exhausting tug-of-war over the definition of their relationship. So while the self-development side (seen from the sender) contains I-messages, the relationship side contains You-messages on the one hand and We-messages on the other. […]

Self-revelation

(or: What I reveal of myself)
In every message there is not only information about the communicated factual content, but also information about the sender themself. We can see from the example that the speaker is obviously German-speaking and probably colourful, in general that it is awake and inwardly present. Furthermore: that he may be in a hurry, etc. Generally speaking: in every message there is a piece of self-revelation by the sender. I choose the term self-revelation in order to include both deliberate self-representation and involuntary self-disclosure. This side of the message is psychologically highly explosive, as we will see. […]

From: Friedemann Schulz von Thun: Miteinander reden 1. Störungen und Klärungen. Allgemeine Psychologie der Kommunikation. Reinbek bei Hamburg (Rowohlt) 1981. S. 30–35.