The Earth is the only planet in our Solar System that supports life.
The Earth is the only planet in our Solar System which supports life.
Which of these two sentences is correct? Should you use that, or which? They both sound...okay,
and the truth is, in everyday language, people use either one. But, technically, it’s THAT.
To see why, you’ll need to understand “restrictive relative clauses.”
A restrictive relative clause contains defining, essential, RESTRICTIVE information about the
noun that comes before it. If you take the clause out, you’ll change the meaning of
the sentence. Let’s try it. The Earth is the only planet in our Solar
System that supports life. The Earth is the only planet in our solar
system. You can see that by taking out the clause
“that supports life,” we changed the meaning of the sentence completely. And the rule of
thumb is: restrictive relative clauses use THAT. NON-restrictive relative clauses use WHICH.
Non-restrictive relative clauses can actually start with which, whose, who, or whom, but
NOT by that.
You use whose, who, or whom when referring to people, and you use which for everything
else. Non-restrictive clauses contain information that CAN be taken out without changing the
meaning of the sentence. Another way to tell it’s a non-restrictive relative clause is
that it is separated from the rest of the sentence by a comma.
For example: The Earth, which is the third planet from
the Sun, supports life. Here you can see that the non-restrictive
relative clause adds information, sort of like an aside, without changing the meaning
of the sentence. Even if you take it out, the main sentence means the same thing:
The Earth supports life. Could you put in THAT in the original sentence
instead of which? No. You cannot start a non-restrictive relative clause with that. Listen to how it
sounds: The Earth, that is the third planet from the
sun, supports life. See? No good.
If you’re still not sure whether to use that or which, try putting the clause in parentheses.
If it works, use which. If it doesn’t, use that.
I served the cakes that I baked for dessert. If we change “that” to “which” and
put the relative clause in parentheses, it looks a little funny:
I served the cakes (which I baked) for dessert. That little pause for the parenthetical phrase
seems odd. The sentence looks and sounds funny, because in this example, “that I baked”
is a RESTRICTIVE relative clause. It restricts which cakes we’re talking about.
I didn’t serve every cake - I served the ones that I baked. Here’s a sentence that
uses a non-restrictive relative clause: The chocolate cake, which was very sweet,
was my father’s favorite. The chocolate cake (which was very sweet)
was my father’s favorite. There already was a little pause from the
commas. That’s a clue that this clause is extra information that can be removed from
the sentence without really changing the message.
Now let’s test ourselves. Which word would you use in the following sentences, that or
which? The letter that you wrote to my sister was
delivered yesterday. The letter which you wrote to my sister was
delivered yesterday. We would use that, because we're not talking
about ALL the letters - just the one you wrote to my sister.
The letter, that was three pages long, made her very happy.
The letter, which was three pages long, made her very happy.
Here, we use which. It's extra information that it was three pages long. You can take
that out, and still have the most important part - that it made her happy.