Which is it?

A short note about the word "which". Like the failure to "it's/its", the misuse of the word "which" is both widespread and easy to correct. It usually comes about in a sentence like this one.

The committee is considering a bill which may substantially raise sales taxes.

The use of the word "which" in this sentence is incorrect. The reason is "may substantially raise business taxes" is a restrictive clause. It modifies its subject "bill" by restricting it to mean one particular bill. If you delete the entire clause from the sentence, then you change the meaning of the word "bill". In contrast, suppose this is the next sentence in the paragraph.

The bill, which is widely unpopular in the business community, would eliminate the groceries tax exemption.

The clause "which is widely unpopular in the business community" is a descriptive clause. It describes its object, "bill", but doesn't restrict it. If the clause weren't in the sentence, the word "bill" would mean the same thing. Since it's a restrictive clause, it has to be set off from its object with a comma, or in this case, two.

There are various terms used to describe these two clauses (for example restrictive and non-restrictive) but the terminology is not as important as the usage and the "which-that" distinction. As I noted earlier, it is easy to eliminate this mistake and, in so doing, improve your written English. It's a process called "which hunting".

After you've drafted your text, search it for every occurrence of "which". Each time you find it, see if it's in a restrictive clause. Here's the test: if you take out the clause beginning with "which", would it change the meaning of the clause's subject? If not, then the clause is only descriptive, and you can -- and should -- use "which". Otherwise, you have to use "that". Also, if it's descriptive, use a comma. If it's restrictive, don't use a comma.

If you do this regularly, you should find that you're changing a lot of whiches to thats. And your readers will think your writing has improved, but they won't be able to figure out why.