Relative markers of the same form as interrogative pronouns are found only in postnominal ad-relatives* (and their right-extraposed** counterparts) and in correlative relative clauses.
Standardized
IF relative marker of the same form as interrogative pronoun is used, THEN relative clause is postnominal or correlative.
1. * ad-relatives are prenominal […S…NP]NP and postnominal [NP…S…]NP relative clauses, in which S is adjoined to the head NP. ** right-extraposed relatives occur at the end of or following the clause containing the modified nominal2. Downing (1977) formulated this statement the following way: “In prenominal relative clauses there is no pronominal form of the relative nominal distinct from forms used in other nonrelative and interrogative functions. That is, if the relative clause is retained, it is retained in a weak pronominal form; it does not have an interrogartive or special relative form.
1. * ad-relatives are prenominal […S…NP]NP and postnominal [NP…S…]NP relative clauses, in which S is adjoined to the head NP. ** right-extraposed relatives occur at the end of or following the clause containing the modified nominal2. Downing (1977) formulated this statement the following way: “In prenominal relative clauses there is no pronominal form of the relative nominal distinct from forms used in other nonrelative and interrogative functions. That is, if the relative clause is retained, it is retained in a weak pronominal form; it does not have an interrogartive or special relative form.