As a writing trainer and editor, I am often asked about the distinction between the words “which” and “that.” Briefly, it is a stylistic preference, not a grammatical rule. Customarily, the word “which” introduces non-restrictive or non-essential information. The word “that” introduces restrictive or essential information. Given this, the word “which” always takes a comma; the word “that” does not.
Here is an example of “which” correctly used:
The article opens with a discussion of the proposed theory of conceptual change, which is grounded in three suppositions. (28)^
Rather than repeat the sound advice currently available about the distinction between the two words, I’d like to use this article to identify some of the finer nuances of their use.
Both words are relative pronouns. Like all pronouns, they must refer to a noun (called the antecedent). The noun to which they refer is normally interpreted by the reader as the nearest noun to the pronoun. This can sometimes cause confusion in scientific and technical writing if the word closest to the noun is not the concept to which the pronouns “which” or that” refer.
Here’s an example:
As largescale development increases, so does noise pollution, even in urban areas that are set aside for wildlife management, which challenges wildlife ecologists to find new solutions.
In the previous sentence, the pronoun “which” technically refers to the noun “management.” However, clearly the author intends the pronoun to refer to the problem of noise pollution. We can revise the sentence by replacing the word “which” with a noun phrase, like so:
As largescale development increases, so does noise pollution, even in urban areas that are set aside for wildlife management, a serious challenge for wildlife ecologists who wish to find new solutions.
The revision is a lesson in logic, requiring the editor or the writer or both to clarify the sentence’s expression. I find this to be a tremendous advantage of thinking carefully about words, their parts of speech, and their exact meanings. Ultimately, such thought leads to more articulate, clearer writing.
At other times, the relative pronouns “which” and “that” do not refer to words per se, but to concepts. At such times, we must summarize or restate the concept. We can do so either by capturing the idea in an –ing verb or by restating the preceding clause in a noun or noun phrase followed with the word “that.” Here are two examples:
Original, flawed sentence:
In the post-test, students’ scores in this section improved to 83%, which indicated that whatever the source or sources of their incorrect answers was in the pre-test, they were corrected by instruction. (38)^
In the preceding sentence, the word “which” does not refer to the 83% score. Instead, it refers to the concept of students’ scores’ improving. Here are two ways to revise:
Revision using an –ing verb:
In the post-test, students’ scores in this section improved to 83%, indicating that whatever the source or sources of their incorrect answers was in the pre-test, they were corrected by instruction. (38)^
By dropping the relative pronoun and turning the following verb into a participle, we eliminate the problem.
Revision by restating the concept:
In the post-test, students’ scores in this section improved to 83%, an improvement that indicated that whatever the source or sources of their incorrect answers was in the pre-test, they were corrected by instruction. (38)^
Note that if you do not wish to sound monotonous, you can always replace the word “improvement” with a synonym, such as “increase” or “progression.” Some people might object to having two “that” clauses in such close proximity to one another. Such stylistic considerations are good reasons to consider all of your options before settling on one.
After you have mastered the distinction between “which” and “that” as restrictive or non-restrictive, start looking at the two words as relative pronouns and think about the words to which they refer. Is that relationship technically correct? In other words, do the words “which” or “that” have clear antecedents? If not, think about which of these strategies you could use to clarify that relationship.
^From Chi, Michelene TH; James D. Slotte; Nicholas de Leedaw. “From things to processes: A theory of conceptual change for learning science concepts.” Learning and instruction. 4(1): 27-43. Retrieved from Google Scholar 31 May 2016. Some quotes modified for instructional purposes.