This one confuses people, but grammar nerds know that it’s simple: You use “that” for restrictive clauses and “which” for non-restrictive clauses.
Huh? What does that mean?
That = restrictive clauses
A restrictive clause is a necessary clause. It narrows down (or restricts) a larger group or category so that it’s clear exactly what you’re referring to.
Ice cream that is contaminated with listeria can cause death.
The “that contains listeria” is necessary. You’re not talking about all ice cream here—just ice cream contaminated with listeria. So you need “that.”
Which = non-restrictive clauses
A non-restrictive clause provides extra information about someone or something. It’s not necessary. It doesn’t help narrow (or restrict) a larger group. It just gives you a little more.
Ice cream, which is America’s most popular dessert, can be found in every grocery store.
You can just as easily write this sentence: “Ice cream can be found in every grocery store.” You don’t need “…which is America’s most popular dessert.” But it’s a nice little tidbit.
A NOTE ABOUT COMMAS
Generally, commas surround clauses that are not necessary—ones that provide extra information about a person or thing. Non-restrictive clauses, which provide extra information, get commas. (See what I did there?)
But restrictive clauses don’t get commas. After all, they’re necessary. You don’t want a comma to block your reader from getting necessary information.
Now, let’s go back to our first example and look at it differently:
Ice cream, which is contaminated with listeria, can cause death.
That is a terrifying sentence. It’s saying that all ice cream contains listeria and can cause death. Yikes! But of course, that’s not true. It’s only ice cream that is contaminated with listeria.
A real example from a client
Let’s look at this issue in a client’s quarterly investment report.
Where nominal short-term interest rates have approached zero, some central banks have resorted to unconventional monetary policies which have massively expanded their balance sheets.
Ignoring other changes I might make to this sentence, let’s address the “which.”
First, let’s identify what it’s describing: unconventional monetary policies.
Now, let’s identify the descriptive clause: which have massively expanded [central banks’] balance sheets.
Then we ask, Do all ‘unconventional monetary policies’ necessarily ‘massively expand central banks’ balance sheets’?
No. Surely, some unconventional monetary policies would not massively expand central banks’ balance sheets. After all, an unconventional monetary policy could be to give away large sums of cash on the city streets, and that certainly wouldn’t benefit a central bank.
Therefore, we need the clause “massively expand their balance sheets.” Without it, we wouldn’t know which “unconventional monetary policies” the sentence was referring to. Just like ice cream contaminated with listeria, we need to know which monetary policies are being discussed.
Since we need the clause, we use that, not which.
So the sentence should be written:
Where nominal short- term interest rates have approached zero, some central banks have resorted to unconventional monetary policies that`have massively expanded their balance sheets.
A quick review
Use that for clauses that are necessary, those that restrict a larger category. Use which for clauses that provide extra information, ones that don’t narrow down or restrict a larger group. And surround a whichphrase with commas.