This is not really a quibble, because it’s okay with me no matter how writers treat this. Care about the difference? Fine. Ignore it completely and use “which” instead of “that”? Also fine with me.
Someone asked me about this on Quora, so I did explain the difference. Luckily I have been pestered into learning this by various copy editors over the years, plus I have figured out an easier way to tell whether to use “that” or “which” so I don’t have to think about this much anymore. I use “that” if there’s no comma before the word and “which” if there is. I don’t think at all about restriction, just about commas, because that is quicker and easier to notice on the fly when you aren’t thinking about grammar, just about getting words in a row.
Here is the actual rule:
For American writing: you are supposed to use “that” when introducing restrictive relative clauses and “which” when introducing nonrestrictive relative clauses.
My cousin has a dog that is bigger than she is. The information about the dog is essential. The clause is restrictive.
My cousin has a dog, which caught a mouse yesterday and brought it into the dining room while her family was having dinner. The information about the dog is not essential. The clause is nonrestrictive.
Restrictive vs nonrestrictive is in many cases an aesthetic judgment, rather than really cut-and-dried. That is why I prefer to consider commas rather than restriction. It’s much easier to decide by feel whether to use a comma, and then decide on that basis whether you should use “that” or “which.” (Obviously you do need a feel for commas first.)
In British writing, you can use “which” in either case. Because I think the concept of restriction is a touch artificial and that many clauses could be viewed either way, I actually prefer the British attitude here, but my copy editors don’t.
You remember those eight historical novels from the previous post? Well, let me point out something from several of them.
This is from White Eagles Over Serbia:
This afternoon in June was something of an exception – and he surprised himself when he found that he was crossing the marble staircase by the porter’s lodge, to push open the swing doors which opened on the private lounge.
Look, no comma before the “which.” Is that clause restrictive or nonrestrictive? I would say this clause is restrictive, so in American writing, the author should have used “that.” But did you, as the reader, notice this use of “which” in that paragraph when I posed all those novel beginnigns? Were you momentarily concerned about whether the upcoming clause was restrictive or nonrestrictive? Any moment of confusion? I would bet not.
Here are a couple of sentences from The Moonstone:
My object is to explain the motive which has induced me to refuse the right hand of friendship to my cousin, John Herncastle. The reserve which I have hitherto maintained in this matter has been misinterpreted by members of my family whose good opinion I cannot consent to forfeit.
Here is a fragment from Mansfield Park Revisited
without its head, his household hardly knew how to go on, or how to resolve the innumerable difficulties and perplexities which his loss had occasioned
I just thought I would point out that all these uses of “which” in both restrictive and nonrestrictive clauses, are absolutely okay, as unnoticeable for American readers as for British readers; also that the distinction between “restrictive” and “nonrestrictive” is honestly sometimes a touch less clear than English grammar books imply. Is the clause restrictive or nonrestrictive in “The reserve which I have hitherto maintained in this matter has been misinterpreted by members of my family”? I think you could argue it either way, largely depending on whether you thought the sentence would be improved by commas around that clause or not. (I agree with Wilkie Collins; that sentence does not need extra commas.)
For those writing for American copy editors, I would not call this a hill to die on, so as far as I’m concerned, you can just let the copy editors change “which” to “that” if they feel so inclined. And learn to do it their way just to be nice and make less work for them. But I do think following the British example would make life easier for us all, without causing any problems with the clarity of writing.