The pronouns that and which tend to trip up writers. Both words introduce clauses, but not the same types of clauses. Not understanding this subtle distinction can trick you into misusing the words.
But don't worry; you can apply a simple reading strategy to help you choose the right word. When a sentence uses a clause that begins with that or which, read the sentence without the clause.
If the clause is essential to the meaning of the sentence, use that.
The agency’s social media policy forbids Twitter posts that include overt political content.
If you remove the bolded clause, the sentence’s entire meaning would change. This makes the clause restrictive. (The clause literally restricts the meaning of the sentence). Always use that to introduce restrictive clauses.
For another example, apply the reading strategy to Jake Nickell's quotation at the top of today's eTip.
Okay, so when should you use which?
If removing the clause does not change the meaning of the sentence, use which.
The Twitter post under investigation, which the writer has since apologized for, included a clear political bias.
Reading the sentence without the clause does not change the overall meaning. The apology is nonessential information (nonrestrictive), so which is correct.
Corralling Clauses with Commas
Notice that commas set off the nonrestrictive clause from the remainder of the sentence. This is another trick that can help you spot errors. Commas set off nonrestrictive clauses but not restrictive ones.
If you notice a dependent clause that begins with which but has no comma, something is wrong. If the sentence makes sense without the clause, keep which and add any needed commas. If the sentence does not make sense without the clause, change which to that.
Incorrect: It was her offhand remark which led to the investigation.
Correct: It was her offhand remark that led to the investigation.
Likewise, if you notice a comma preceding a clause introduced by that, you likely discovered an error.
Incorrect: It was her offhand remark, that led to the investigation.
Correct: It was her offhand remark that led to the investigation.
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