I’m not Grammar Girl, but I like to listen to her. I am also particular about language. Somewhat. I have my idiosyncrasies, just like everyone else. Take, for example, the spelling I employ on this blog. Is it Canadian English or US English? That’s a great question. There’s not a right answer. It’s a conglomeration. If that was one of my pet peeves, I would be driven nuts by this blog. But, it’s not and I’m not concerned about it. Sorry.
However, you want to get me going on something? I just read a book that featured the improper use of the word Which ninety percent of the time it was used. Yeah, that will get me going.
The recommended usage of which is to set off nonrestrictive clauses in sentences, while that is used for restrictive clauses. The Hermit Editor famous in the comments will tell you that a clause is restrictive if the sentence cannot be understood without it. A restrictive clause is not set off with commas. A nonrestrictive clause can be left out without damaging the sentence. Nonrestrictive clauses are set off with commas.
So, an example:
Standing in the dog park, he said, “The dog that has three legs belongs to me.” “That has three legs” is essential to understanding the sentence. Without it, the sentence becomes “The dog belongs to me.” Fine if there’s only one dog present, but at a dog park, you’ve probably got to specify.
Alongside the officer trotted a toy poodle, which is hardly a typical police dog. “Which is hardly a typical police dog” is not essential to understanding the sentence. Of course, it hardly needs to be said (except maybe for humorous effect), but if you’re going to say it, you use which.
If I have saved one use of the word which from being mangled, this post will have been worthwhile. Also, if you’re British, ignore this post. You were already planning to, I know. (Which example borrowed from The Chicago Manual of Style, 15th edition, 5.202).