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When should you say that, and when should you say which?

Don’t make the mistake of saying which when you mean that.


This is an entry in an ongoing series of examinations of writing mistakes people frequently make and tips on how to be a better writer. 

While proofreading, one of the most common mistakes I encounter is writers incorrectly using the word which when they mean that.

For example:

“Workfare will result in an increase in employment opportunities over those opportunities which would otherwise be available.”

The problem is that the sentence uses the word which within in a restrictive clause. In other words, “which would otherwise be available” is essential to the meaning of the sentence—in order to properly communicate the meaning of the sentence, the clause must be included. And because it’s a restrictive clause, it should use the word that, not which.

Is this really a rule? It seems silly. 

Is this arbitrary? It is indeed! But aren’t all rules of writing and grammar arbitrary? What makes them work is that they are widely accepted and adopted. For that reason, that is used in sentences that contain restrictive clauses, while which is used in sentences that contain non-restrictive clauses—parts of the sentence that can be removed without altering the sentence’s meaning.

Moreover, the word which is always preceded by a comma.

The incorrect switching of that and which can cause utter confusion and chaos. See, for example, these two sentences:

My laptop that is missing a space bar is on my desk.

My laptop, which is missing a space bar, is on my desk.

In the first sentence, the implication is that I own more than one laptop, and I am distinguishing one laptop from its laptop brothers and sisters. In the second sentence, I am merely adding some extraneous details about the machines, details that could easily be cut from the sentence entirely.

If this is the rule, then why do so many people get it wrong?

The problem is that it was not the rule until the 20th century. Up until then, that and which were generally used interchangeably when introducing restrictive clauses. But in 1906, the Fowler Brothers published The King’s English, the foundational text for modern English grammar. In it, the Fowlers standardized the proper uses of that and which.

That’s why so many pre-20th-century texts use which in ways we’d now deem incorrect. For example, the United States Constitution:

But this long history of which-usage does not die off easily. Modern writers either don’t know about the rule or they don’t care.

The only thing scary about this Stephen King book is its not respecting restrictive clauses.

Can you just sum this all up for me so it’s easy to remember?

Sure! Here’s what you need to know: That is used in restrictive clauses—when the information is necessary, go with that. When it’s not, and when you use a comma, go with which.

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