The rose that is in the garden is red.
The rose, which is in the garden, is red.
Do you see the difference?
The rose that is in the garden is red. In the first sentence, the rose that is in the garden is red, as opposed to, say, the rose that is in the vase on the mantelpiece, which could be red too, though that is not specified; the other rose could be white, or yellow, but that’s immaterial, because we’re talking about the rose in the garden. We use that because the fact that that rose is in the garden is crucial to clarifying which rose we are talking about. That garden needs to be bolted to that rose, so we use that, and a that, at that(!), that’s unseparated from the rose by commas. The clause ‘that is in the garden’ is a restrictive clause, i.e., it restricts or specifies meaning, and our understanding of that sentence fundamentally depends on it: we’re talking about the rose in the garden.
The rose, which is in the garden, is red. In the second sentence, the fact that the rose is in the garden is supplementary information – extra description, perhaps, but not essential to our understanding. The core meaning of that sentence is that the rose is red. The fact that it is in the garden is secondary. You could cut it and bring it indoors and we could then say: The rose, which is now in the vase, is red. The fact that it was previously in the garden remains secondary; the fact that it is red is foremost. The clause ‘which is in the garden’ is a nonrestrictive clause, i.e., it does not restrict or specify meaning, and our understanding does not depend on it; we could in fact remove that clause and we’d still understand the basic meaning of that sentence: the rose is red (and it just happens to be in the garden).
Also note that the nonrestrictive which clause is set off by commas; it can be whipped out of the sentence, along with those commas, because it’s not essential.
For years, I was lost. I did not know the difference, could not see the difference, between the relative pronouns that and which. My lack of formal grammar training (back then) and my reliance on gut instinct left me floundering (I had seen many whiches that I was told should be thats – but why, o why?!). My friend Helen at work tried to explain, but I was too stupid (she’d been to Oxford and had a degree in German and Russian; I’d been to Hull and had a degree in American Studies).
Then one day, it just dawned on me. Ding! And now I get it. Not so stupid anymore.
This distinction is something that is not always easily grasped. Like many matters that are so often a matter of instinct, you need to hear it time and again, and in different voices and from different perspectives, before it sinks in. A further useful explanation, I find, is the one given by Bill Bryson in his Dictionary of Troublesome Words.
Of course, that is a word that also serves other functions: demonstrative pronoun (That is mine), demonstrative adjective (That man), subordinate conjunction (I am so happy that I could cry). So you can end up with sentences like ‘I noted that that that he talks about is rubbish’. I have to say that in a perverse way I really quite like that. Oh, that word again.
Does this matter? Grammar descriptivists would say not. Meaning is clear, they’d say. But is it, always?
I’m not really a prescriptivist; I far prefer the notion of usage to the idea of rules. I once had a lecturer whose books were full of whiches where I would now put thats. Never mind, and we all survived (and the lecturer clearly was not stupid; he ended up a professor at Cambridge – sorry, Helen, I know it’s the other place, but). Some writers simply prefer the rhythm of a which (though I can no longer hear that music, I’m afraid), while others propose that UK practice often favours which where the US would insist on that, a point supported by Oxford Dictionaries.
And a lot of publishers and editors do not worry about that vs which, though I have known copyeditors who’ve got in trouble for changing whiches to thats, so clearly someone worries, and some editors are even stirred to action. The wonderful Grammar Girl tells us that:
In fact, having a client try to overrule my correction of a which to a that was one of the things that pushed me over the edge and made me start the Grammar Girl podcast.
But various usage guides and other authorities tell us not to fuss, either. There are more important things to concern ourselves with. (Like the freedom to end sentences with prepositions.)
However, there can be a clear distinction in the use of that and which, and there is a beauty in the availability of such clarity. Some of us do care, even in the UK.
I recently copyedited a work in translation, and I changed a which to a that. When I checked over the translator’s responses, I saw he’d flagged the margin with an ‘EEK!’. Eek, said I back; was this an eek of horror at my purism, or his oversight?! I double-checked, and at this point he told me of his ‘own insistence on this useful distinction (sadly in abeyance here). It shocks me that so many really fine writers seem unaware of it. It can radically alter meaning.’
Of course late at night, updating my blog, it’s hard to come up with examples. But we know you’re out there! And I’m going to start collecting examples of such instances. (Updated 23 September 2013.)
March 2019 update: Things change, life moves on, and I’m not sure I would be so ardent in changing whiches to thats nowadays! Though I do still prefer this distinction in my own writing, today, if I were editing I’d simply ask writers if they prefer to follow the difference themselves. Have I just grown slack? Or maybe it’s simply that I’ve seen so many whiches in place where they really don’t hurt that I figure it’s best to leave them as they came out.