A structure is an organic whole, in which the different elements play different roles.
- Halliday (1985), a pag.26

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. Each of these kinds of meaning is expressed by means of certain configurations of functions. Thus Theme, Subject and Actor do not occur as isolated; each is associated with one or more other functions of the same kind, together with which it forms meaningful configurations. A meaningful configuration of functions of the same kind is what is meant by a STRUCTURE. Thus the functional label has no significance in itself. Its significance lies in its relationship to other functions with which it is structurally associated; the total structure is what expresses, or REALIZES, the meaning intended by speaker or writer [...] Actor and Process together form a structure, one of a number of possible structures through which the clause expresses one particular kind of meaning. In similar fashion both Subject and Theme enter into configurations with other elements, forming structures which express meanings of other kinds.
- Halliday (1985), a pag.37

This book is not an account of the theory, nor does it present system networks for the grammar of English. It presents the structures wich are the output of the networks – which realize, collectively, the sets of features that are chosen [...] Structure is an output device, the mechanism for expressing the choices that have been made.
- Halliday (1985), a pag.XXVII