7.3.4 The interaction between wh-movement and inversion

This is a puzzle for which I have no real account. Apparently it is a very general condition that a CP can contain either an overt operator or an overt complementiser. We will see that this extends to other clauses too. Moreover, it appears not to be violated by an auxiliary moving to C. The condition has been known as the Doubly Filled COMP Filter since (1977) when it was introduced by Chomsky and Lasnik. However, this stipulatory account has never been superseded by anything more explanatory. I will therefore adopt the Doubly Filled COMP Filter as a condition on the well-formedness of structures in lieu of a proper explanation: