Both theme and information are realized as configurations of structural functions, though with two important differences between them. Theme is a system of the clause; and it is realized by the sequence in which the elements of the clause are ordered – Theme comes first. Information is not a system of the clause: it has its own domain, the information unit, which typically corresponds to a clause but not necessarily so; and it has its own realization in the form of tonic prominence – which typically comes at the end of the information unit, but again not necessarily so. It is these differences that make it possible for thematic and informational patterns to be combined in so many varying ways. Theoretically, there is no reason why Given and New should not also have been organized as a system of the clause and realized by the sequences of the elements – like Theme and Rheme, only based on final instead of initial position, with the New always coming last.
- Halliday (1985), a pag.287