Clauses referring to a proposition serve to specify the content of a head noun. The propositional clause is obligatorily introduced by a complementizer and does not contain a gap. Propositional clauses may be restrictive or non-restrictive. If they are restrictive, propositional clauses function as complements of the head noun and will be treated as such in Section 15.3. This section will therefore only deal with non-restrictive propositional clauses such as (554).

As in the case of propositional complement clauses, non-restrictive propositional clauses only follow a noun denoting a proposition. As can be seen from the examples in (554), the antecedent of non-restrictive clauses is typically definite. This is to be expected, since the use of a non-restrictive modifier suggests that the message, announcement, etc. conveying the proposition can be regarded as known to the addressee, or at least identifiable from the context. However, it is also possible for the head to be indefinite. This happens especially when there is uncertainty about the correctness of the specified content. In the constructions in (555) this uncertainty is expressed by the (obligatory) use of the modal zoudenwould and the use of the subordinating conjunction als instead of dat.