A Which Hunt*
That and which both can be used as pronouns at the opening of relative clauses: “that the boys looked into” or “which she threw.”
When that is used in this way, the resulting clause is normally considered to be restrictive — it is needed for an understanding of the central meaning of the sentence.
When which opens a relative clause, the resulting clause may be treated as restrictive or non-restrictive.
In common practice, that emphasizes and which de-emphasizes. Again, to emphasize or to de-emphasize — that is the question. Consider the following sentences.
The coffee, which was old, sat in the pot.
The coffee which was old sat in the pot.
The coffee that was old sat in the pot.
In the first sentence the relative clause is de-emphasized for two reasons: first, the clause is set apart by commas; second, the clause opens with which. The major point of the sentence is simply that the coffee sat in the pot. In the third sentence the relative clause is emphasized. The use of that and the choice not to set it apart with commas suggests a sentence with two main points: the coffee was old and it sat in the pot. Both points are important. The second sentence presents an author’s middle ground. The use of which de-emphasizes the clause, but the failure to use non-restrictive commas adds weight to the idea within the clause. (You should know that a minority of writers see no difference between that and which and use them interchangeably.)
We might, of course, reword the last example to achieve a less wordy and more effective sentence.
The old coffee sat in the pot.
Below is a sentence that is working at cross purposes and is thus incorrectly punctuated”
The marble, that the cat hid away, was round.
*Why a “Which hunt”? As a graduate student I took a very difficult course in deconstructionist literary theory (difficult for me). I seldom had a clue what Wendy Steiner, the professor, was talking about. At one point during the semester she handed back an essay and stated, “You need to go on a witch hunt.” That, at least, is what I heard. Knowing how lost I was in the course, and fearing that I had missed a significant point of theory, I plucked up my courage and asked what she meant by that comment. She replied, “You do not know the difference between that and which.” She was right. I’ve been hunting ever since.
Worksheet practicing that vs. which