[...] Relatives do not form a separate word class; they are either nouns or adverbs. They function as Subject, Adjunct or Complement; either alone, or within the structure of a group, nominal or adverbial, or a phrase [...] Relatives are thus like WH- interrogatives, in that they have both a function in the clause (as Subject, etc.) and a special status of their own. They are of course closely related to interrogatives [...].
- Halliday (1985), Pag. 52