The Theme is the element which serves as the point of departure of the message; it is that with which the clause is concerned. The remainder of the message, the part in which the Theme is developed, is called in Prague school terminology the Rheme.
- Halliday (1985), Pag. 38

There is a close semantic relationship between information structure and thematic structure [...] Other things being equal, a speaker will choose the Theme from within what is Given and locate the focus, the climax of the New, somewhere within the Rheme. But although they are related, Given + New and Theme + Rheme are not the same thing. The Theme is what 'I', the speaker, choose to take as my point of departure. The Given is what 'you', the listener, already know about or have accessible to you. Theme + Rheme is speaker-oriented, while Given + New is listener-oriented.
- Halliday (1985), Pag. 278