Complementizers include WH- words and expletives that, if, and whether, all of which can be used to form subject or object relative clauses.
In the following examples, the rule of ordinary relative clause formation is such that the complementizer (bolded) is identical to the noun in the matrix noun phrase (italicized). As a result of this rule acting upon the relative clause, the last word, which has been pronominalized, is to be deleted because it is unnecessary.
The head-initialized antecedent (bolded), which is a complementizer and the syntactic head of a full clause, is an object of the embedded clause (underlined):
SUMMARY/OBSERVATIONS: Pronominalization (using a pronoun instead of a noun) is a standard approach to relativization, whereby a noun or noun phrase in an embedded sentence is deleted or pronominalized.