The complement clause and the relative clause share a negative property: both are subordinate clauses that bear no specific interpropositional relation to anything in the superordinate clause. Therefore the structural means that subordinate them are often similar or even identical. Doubts may therefore arise in the analysis of specific cases.

There is a semantic basis to the distinction:

• A complement clause designates a proposition, i.e. something that can be true or false.
• A relative clause designates an object (or set of objects) of a certain kind, viz. an object fulfilling a certain participant role in it.

From this results the first criterion by which they are distinguished. It concerns the selection restrictions of the clause component that the subordinate clause depends on:

• If this component takes a propositional argument, the dependent clause is a complement clause.
• If this component takes a concrete argument, the dependent clause is a relative clause.

In some cases, this criterion suffices to make the distinction:

The dependent clause in is a complement clause because find out takes a propositional object. The dependent clause in is a relative clause because find takes a concrete object.