which referring to a clause or sentence. The relative pronoun which can sometimes refer to a clause or sentence, as opposed to a noun phrase: She ignored him, which proved to be unwise. They swept the council elections, which could never have happened under the old rules. While these examples are perfectly acceptable, using which in this way sometimes creates ambiguities. The sentence It emerged that Martha made the complaint, which surprised everybody may mean either that the complaint itself was surprising or that it was surprising that Martha made it. You can avoid the ambiguity by using other constructions such as It emerged that Martha made the complaint, a revelation that surprised everybody. It is important to remember that you can use which in this way only when the clause or sentence it refers to precedes it. When the clause or sentence follows, you must use what, particularly in formal style: Still, he has not said he will withdraw, which is more surprising. Still, what is more surprising, he has not said he will withdraw.
