When I read this tweet this morning, I knew that “which” was the wrong relative pronoun and should be replaced by “that”. However, my years of having two hours of English grammar per week are but a distant memory, and I couldn’t quite remember the grammatical rules that govern this particular use of which and that. Being a bit of a grammar geek enthusiast, I thought I’d remind myself how it all works.
It’s all to do with restrictive and non-restrictive relative clauses, of course! Restrictive relative clauses contain essential information, without which the sentence doesn’t make sense, and they can be introduced by that, which, whose, who or whom.
Example: We have got to clean the stinking swamp that is the House of Lords.
On the other hand, non-restrictive relative clauses give extra information, without which the sentence still makes sense. They can be introduced by which, whose, who or whom, but NOT by that, and should be separated from the main clause by a comma.
Example: We have got to clean out the stinking swamp that is the House of Lords, which is only marginally more repulsive than the House of Commons.