'That' and 'Which' 99

Anyone who’s followed the “Associated Press Stylebook” or the “Chicago Manual of Style” has been told, in no uncertain terms, that 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.”

Style books do a good job explaining what, exactly, nonrestrictive and restrictive clauses are. I’ll get to that in a second. But what they fail to explain is what a style book is and what its “rules” really mean.  As a result, a lot of people think there’s a rule against “which” in sentences like the one above when, in fact, there is not.
A style guide, as I’ve said before, isn’t a universal rule book.  It’s more like a playbook -- a “here’s how we do it” as opposed to “here’s the only right way do it.” That distinction has eluded a lot of people, including me for some years. But it’s an important one.

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.

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.