That which confounds us…
I received my copyedited manuscript back from my publisher, and am proud to say that it was VERY clean, except that I seem to completely miss the difference between which and that. Something about restrictive and non-restrictive clauses, but that means nothing to me. Can you explain? – David, author
Restrictive and non-restrictive clauses are adjective clauses – they modify or describe the noun, either the subject or object, of the sentence. They’re also called relative clauses because they relate to the rest of the sentence.
Restrictive clauses do just what they say: they restrict the subject or object. They add a specific description to distinguish the subject or object from any other. Removing the restrictive clause from the sentence would change the meaning of the sentence or make it confusing or too vague.
Non-restrictive clauses add additional information about the subject, but they don’t restrict the subject to a specific item. The non-restrictive clause can be removed from the sentence without changing the meaning.
Makes perfect sense, right? Ahem. Okay, it makes more sense with examples.
The car was stolen.
‘The car’ is the subject of the sentence. It was stolen.
What car was stolen? We need more information.
The car, which was in the parking lot, was stolen.
That is a non-restrictive clause. There may have been lots of cars in the parking lot. If you remove this non-restrictive clause, it doesn’t change the meaning of the sentence. The car was stolen. The non-restrictive clause (‘which was in the parking lot’) adds additional information, but doesn’t restrict it to one specific car.
The car that was in the disabled parking space was stolen.
Oh, that car. “That” added more information that specifically restricted the subject of the sentence to that car. Not any of the other cars in the parking lot. That car. That car that used to be right there but isn’t there anymore because it’s been stolen.
Non-restrictive clauses (‘which’ clauses, adding more info but not restricting) are set off with commas from the main sentence because it is additional information, almost a parenthetical statement. The car, which was in the parking lot, was stolen.
Some ways it can get mixed up:
I love pasta, which this restaurant serves. (non-restrictive)
“Pasta” is the object of this sentence. This sentence means I love pasta in general. Some added information is that this restaurant serves pasta, but I’m not specifically saying I love the pasta served at this restaurant – I’m not restricting my statement to only pasta from this restaurant. In fact, my next sentence might say, “However, this restaurant’s pasta tastes like crap.”
I love the pasta that this restaurant serves. (restrictive)
This means I specifically love the pasta served at this restaurant. I may or may not love pasta from other restaurants -- I am restricting the meaning of my statement to refer specifically to the pasta served here.
A way to check: if you can eliminate the word ‘that’ and it still makes sense and is grammatically correct, then it’s a restrictive sentence and you should use ‘that,’ not ‘which.’ Or, even better, you can just eliminate ‘that’ to save on word count and tighten your sentences. I’ve edited novels that contained 1,000 unnecessary ‘thats.’
I love the pasta this restaurant serves.
This sentence removes the word ‘that.’ It’s still grammatically correct, and the meaning of the sentence remains the same. I could use the word ‘that’ if I wanted it, but the sentence stands without it.
WRONG: I love the pasta which this restaurant serves.
‘Which’ is non-restrictive so it needs a comma to set the clause apart as additional, separate information.
WRONG: I love the pasta, that this restaurant serves.
‘That’ is restrictive, so there shouldn’t be a comma in this sentence. If you eliminate ‘that’ from this sentence you wind up with an errant comma and an incomplete sentence: I love the pasta, this restaurant serves.
If you’re referring to people, instead of using that/which, use who/whose. Who/whose can be either restrictive or non-restrictive, so it's the commas setting the clause apart that signal to the reader whether it's restrictive or non-restrictive.
The scientist who discovered gravity was named Newton.
This sentence is restrictive, so no comma. I’m referring specifically, restrictively, to the one scientist who discovered gravity. And yeah, don’t quibble on my science here. But here we use ‘who’ instead of ‘that’ because it refers to a person rather than a thing.
My favorite scientist, who discovered gravity, is Newton.
This sentence is non-restrictive. Take out the clause and it still make sense: My favorite scientist is Newton. The non-restrictive clause adds additional information. Newton is my favorite scientist, oh and by the way, he discovered gravity. Here, we use ‘who’ rather than ‘which.’
My favorite scientist, whose discoveries included gravity, is Newton.
Non-restrictive clause adds more information to this sentence, but it still makes sense without the clause. The clause is set off by commas, uses ‘whose’ (possessive) instead of ‘which’ because the clause refers to a person rather than a thing.
My favorite scientist who discovered gravity is Bhaskaracharya.
Restrictive clause. Two scientists discovered gravity, and this sentence restricts my favorite to only one of them.
As we’ve seen above, when it’s a restrictive clause, you can often eliminate the word ‘that.’ Sometimes you can change the verb in the restrictive clause to an -ing verb and eliminate the ‘that.’ For example:
I watch the whales that swim next to the boat.
Restrictive ‘that,’ no commas, specifies exactly which whales I watch. I watch those whales that swim next to the boat. I do not watch the whales that swim a mile away. I watch those whales right there.
I watch the whales swimming next to the boat.
Restrictive, same as above, but drops ‘that’ and changes ‘swim’ to ‘swimming.’
WRONG: I watch the whales, swimming next to the boat.
Does not take a comma here as it is a restrictive clause. Also, that little comma confuses the whole meaning of the sentence. Does it now mean I am swimming next to the boat while I watch whales?
Got all that?
Use ‘that’ without commas to restrict the subject or object of the sentence to a specific one. The sentence won’t make sense without the restrictive clause.
Use ‘which’ with commas to add extra information about the subject or object of the sentence. Sentence will still make sense without the clause.