As an editor, I have come to accept that this is a tricky concept for the average person … and the not-so-average person, too! It might even be the issue I spend the most time correcting in every document that (not which) comes before me. Part of the problem comes from the fact that each of these two words has multiple uses.
That can be:
- subordinate conjunction – He asked that the blue car be washed.
- demonstrative pronoun – That was dirty.
- definite article – That blue car was washed.
- adverb – The car wasn’t that dirty.
- relative pronoun – The blue car that was dirty has been washed.
Which can be:
- interrogative pronoun – Which car has been washed?
- relative pronoun – The blue car, which was dirty, has been washed.
Our concern is with the relative pronouns. The misuse of that and which as relative pronouns is so proliferate that even the highly educated among us question which is correct. Every book or website about grammar has pages of examples and explanations, which can be confusing. They use terms like Restrictive Clauses and Non-restrictive Clauses (because the average Joe knows what that means, right?) Yikes.
If we look at the examples of relative pronouns above, it is the punctuation that gives it away. That never uses a comma; which always does. Simple, right?
The blue car that was dirty has been washed.
- In other words, it was specifically the dirty blue car, not the clean blue car, that was washed. This is an example of a restrictive clause. It is restricted only to the blue car that was dirty, not just any blue car.
The blue car, which was dirty, has been washed.
- In other words, the blue car that has been washed also happened to be dirty. This is non-restrictive, and we could get rid of the extra information between the commas and still have a complete thought. It is not restricted to being dirty.
Let’s take a more complicated example and show why this distinction is so important:
Premier Smith opposes new provincial taxes which will increase the cost of doing business in Saskatchewan.
This example is incorrect because it creates ambiguity, and the reader can’t tell which of the following meanings the author really intended:
Premier Smith opposes new provincial taxes, which will increase the cost of doing business in Saskatchewan. (It is Premier Smith’s opposition to the taxes that will cause an increase to the cost of doing business.)
Or
Premier Smith opposes new provincial taxes that will increase the cost of doing business in Saskatchewan. (Premier Smith specifically opposes any taxes that will increase the cost of doing business, as opposed to taxes that won’t cause an increase.)
Hopefully that clarifies things a little.
Now, don’t get me started on the misuse of You and I vs. You and me!
References
- Einsohn, Amy, The Copyeditor’s Handbook, 2nd ed (Berkeley: University of California Press, 2006)
KJL Editorial 