Do you know the difference between the words "that" and "which"? In common use, they appear to be synonyms, but grammatically, substituting one word for the other can change the meaning of a sentence. Here's a situation where one might confuse the two words:
Sandra placed the pamphlet on the desk that was made of solid oak.
Sandra placed the pamphlet on the desk, which was made of solid oak.
These sentences are deceptively similar. In either case, Sandra's pamphlet ends up on an oaken desk. So, what's the difference?
The first sentence uses the relative clause "that was made of solid oak." This clause is restrictive because it specifies precisely the desk where Sandra's pamphlet lands. In our example, we should use the first sentence to clarify to the reader which desk has the pamphlet. Restrictive clauses beginning with the word "that" do more that describe additional attributes; they tell us where to look for the action in the sentence.
The second sentence uses the relative clause "which was made of solid oak." Unlike the first sentence, this clause is non-restrictive. The word "which" describes additional attributes of the desk. In proper usage, non-restrictive clauses convey related, ancillary information about the subject. The word "which" is inappropriate to convey to the reader that pamphlet is on an oak desk, rather than a metal desk. This is the role for "that." On the other hand, "which" is a fine choice to convey the idea that the desk is made of oak.
Knowing the difference between "that" and "which" can also help with punctuation. Restrictive clauses describe a key point. Therefore, they cannot be dropped without losing valuable information. Do not place a comma before a restrictive clause like "that was made of solid oak." The comma says we can do without what follows and still get the point of the sentence. If what follows is restrictive, this is not so; the whole point of the sentence is tied up in the restriction.
On the other hand, non-restrictive clauses usually contain non-essential information. Therefore, separating these ideas from the main content of the sentence is a good idea. There we have it: the difference between restrictive and non-restrictive clauses is why it is appropriate to place a comma before "which," but not before "that."
The next installment of the Elements of Style Series, "However, but nevertheless" will appear on this blog on Friday, 8 May 2009. I will continue this series each Friday until I run out of interesting topics in grammar and writing.