Can You Use 'That' and 'Which' Interchangeably?

 

Anyone who’s followed the Associated Press Stylebook or the Chicago Manual of Style has been told there’s an important difference between “that” and “which.”  The relative pronoun “that” is for what are called restrictive clauses while “which” is for nonrestrictive clauses. For example, “Hand me the pen which I like” should be “Hand me the pen that I like.”

But style guides aren't grammar books and the rules within aren't necessarily universal grammar rules. This is one of those rules.

If you’re writing or editing according to AP or Chicago style, you should observe their distinction between “that” and “which.” But if you’re not, well, then you don’t have to worry about it. Here is the difference.

A restrictive clause, also called an “essential” or “defining” clause, narrows down the thing it refers to. Compare:
The cars that have flat tires will be towed.

with

The cars, which have flat tires, will be towed.
In the first example, the “that” clause actually narrows down which cars we’re talking about. Only the ones that have flat tires will be towed. In the second example, all the cars will be towed. The “which” clause lets us know that they all have flat tires. But that in no way separates the ones to be towed from others. “The cars” will be towed. All of them. Period.

In other words, a restrictive clause restricts its subject. A nonrestrictive clause does not: It can be lifted right out of the sentence without losing specificity of your subject.
That’s the basic difference between restrictive and nonrestrictive clauses. But, despite what some style guides say, there’s no rule that says you can’t use “which” for a restrictive clause. British speakers especially do it all the time. “The teeth which are causing him the most pain will be extracted.”

If I were editing an article with that sentence in it, I would change the “which” to “that,” only because I work in AP style. But I certainly wouldn’t call it wrong.

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