Theme In Finite Dependent Clauses

Halliday & Matthiessen (2014: 126-7):

If finite, these typically have a conjunction as structural Theme, e.g. because, that, whether, followed by a topical Theme; … If the bound clause begins with a WH- element, on the other hand, that element constitutes the topical Theme … The reason for this, as we have seen, is that the WH- element also has a function in the transitivity structure of the clause.