Halliday & Matthiessen (1999: 529-30):
But the "Theme + Rheme" configuration becomes a message only when paired with another one, that of "Given + New". This construes a piece of information from the complementary point of view, as something having news value — something the listener is being invited to attend to. It may not contain anything the listener has not heard before; a great deal of "news" is totally familiar, being simply contrasted or even reiterated. On the other hand, the entire message may consist of unknown information, for example the first clause of a piece of fiction.
But the message is construed along prototypical lines as an equilibrium of the given and the new, with a climax in the form of a focal point of information: 'this is to be the focus of your attention'. This focal point usually comes at the end; but (unlike the Theme + Rheme) the Given + New structure is not signalled, in English, by word order — it is signalled by intonation, and specifically by pitch prominence, the point of maximum perturbation (falling, rising or complex) in the intonation contour. The principle behind this is clear: if the Theme always came first, and the New always came last, there would be no possibility of combining them; whereas one powerful form of a message — powerful because highly marked — is that in which the two are mapped on to one another, as in no wonder they were annoyed (where the focus is on the interpersonal theme no wonder).