7.3.1 Basic positions within the CP

It makes perfect sense that wh-questions and yes–no questions involve the complementiser system as complementisers contribute the semantics of force to the sentence, but there is independent evidence for the validity of these analyses. To start, that inverted auxiliaries occupy the C position is supported by the observation that inverted auxiliaries and complementisers are in complementary distribution. The best evidence for this involves not interrogative clauses, though it is also true that we never get inverted auxiliaries and complementisers together in an interrogative, but conditional clauses. In English there are two types of conditional clause, one formed with an if complementiser and one formed with an inverted auxiliary: