7.4.2 A comparison between relative and interrogative clauses
Again this may be entirely due to differences in the use of these constructions. As a relative clause modifies a noun making use of an anaphoric operator, there must be an antecedent for the operator to take its reference from. This antecedent, i.e. the modified noun, can provide us with the content of the operator and hence this is recoverable even if we cannot see the operator itself. With a question, however, as there is no antecedent, the content of the operator has to be visible on the operator itself and hence the null operator cannot be used in this way. The null operator used in yes–no questions is clearly non-referential and hence has no specific content to be recovered. In this situation then the null operator can suffice.