Non-Topical Themes And The Markedness Of Topical Theme

Halliday & Matthiessen (2014: 111):

We could set up a paradigm as follows, showing the effect of different initial selections in the clause:

1. no non-topical Theme, 
2. with inherently thematic non-topical Theme,
3. with characteristically thematic non-topical Theme;

it will be seen that the marked topical Theme becomes as it were more and more marked at each step.