These three headings – clause as message, clause as exchange, clause as representation – refer to the three principal 'kinds of meaning' that are embodied in the structure of a clause.
- Halliday (1985), a pag.37

[...] We introduced the notion of a clause as a unit in which meanings of three different kinds are combined. Three distinct structrures, each expressing one kind of semantic organization, are mapped on to one another to produce a single wording.
- Halliday (1985), a pag.38

[…] a clause is the product of three simultaneous semantic processes. It is at one and the same time a representation of experience, an interactive exchange, and a message.
- Halliday (1985), a pag.53

The Theme-Rheme structure is the basic form of the organization of the clause as a message. Within this, the Theme is what the speaker selects as his point of departure, the means of development of the clause. But in the total make-up of the Theme, components from all three functions may contribute. 'There is always an ideational element in the Theme'. There may be, but not necessarily, interpersonal and/or textual elements as well. The typical overall sequence of these elements is: textual ^ interpersonal ^ ideational. (The circumflex ^ means ʻfollowed byʼ.) The sequence textual ^ interpersonal may be modified, which has the effect of being ʻmarkedʼ; but the ideational element is always the final one – whatever follows the first ideational element of the clause is automatically part of the Rheme.
- Halliday (1985), a pag.53-54

Simultaneously with its organization as a message, the clause is also organized as an interactive event involving speaker, or writer, and audience.
- Halliday (1985), a pag.68

We now come to the third aspect of the meaning of the clause, its meaning as representation. Usually when people talk about what a word or a sentence ʻmeansʼ, it is this kind of meaning they have in mind – meaning in the sense of content. [...] We were looking at the clause from the point of view of its interpersonal function, the part it plays as a form of exchange between speaker and listener [...] We shall be concerned with the clause in its ideational function, its role as a means of representing patterns of experience.
- Halliday (1985), a pag.101

A fundamental property of language is that it enables human beings to build a mental picture of reality, to make sense of their experience of what goes on around them and inside them. Here again the clause is the most significant grammatical unit, in this case because it is the clause that functions as the representation of processes [...] Our most powerful conception of reality is that it consists of ʻgoings-onʼ : of doing, happening, feeling, being. These goings-on are sorted out in the semantic system of the language, and expressed through the grammar of the clause.
- Halliday (1985), a pag.101

Parallel with its evolution in the function of mood, expressing the active, interpersonal aspect of meaning, the clause evolved simultaneously in another grammatical function expressing the reflective, experential aspect of meaning. This latter is the system of TRANSITIVITY.
- Halliday (1985), a pag.101