Since being given a thorough thumping by Professor Geoffrey Pullum in 2008, I have not returned to the which thicket, but a former Sun colleague now operating elsewhere has called for assistance. His shop includes editors from different backgrounds who do not agree on that/which usage.
In 1926 H.W. Fowler suggested in Modern English Usage that it would be a good thing to use that for restrictive clauses and which for nonrestrictive clauses.
We interrupt continuity to discuss the vexatious terminology. You may have been taught different terms. Fowler referred to “defining clauses.” You may have been taught “restrictive,” “limiting,” or “essential” as the terms for clauses that limit meaning, identifying one out of two or more possibilities, and “nonrestrictive,” “non-limiting,” or “non-essential” for information that is merely additional or parenthetical.
Restrictive: “The people that walked in darkness have seen a great light.”* Not all people, but the specific class of people who have been in darkness. Nonrestrictive: “Jainism, which was born at about the same time as Buddhism, has had a great impact on Indian culture.” The coincidental rise of Buddhism is not an essential element of the sentence. The former class of clauses is not set off by commas; the latter is.
To get back to Fowler’s distinction, the first thing to stress is that this is not a rule. He merely offered that observing it would be a “gain in both lucidity & ease,” a recommendation that Garner’s Modern American Usage stoutly maintains.
The distinction is frequently, but not universally, maintained in American English, especially in written English, but British and Commonwealth writers continue to use which in both restrictive and non-restrictive senses, and nobody complains.
Merriam-Webster’s Dictionary of English Usage, after a discussion of the historical switches back and forth, comes to this:
We conclude that at the end of the 20th century, the usage of which and that—at least in prose—has pretty much settled down. You can use which or that to introduce a restrictive clause—the grounds for your choice should be stylistic—and which to introduce a nonrestrictive clause.
So if you are writing for American readers, observing the that/which distinction is a safe and advisable course. But unless you can identify some actual ambiguity that would lead to misunderstanding, it’s not a matter worth fretting over.
I am a little disconcerted, however, to see in newspaper journalism increasing instances of that clauses that are plainly nonrestrictive. Perhaps it is just another example of the carelessness and sloppiness characteristic of journalism. But – I want to be charitable – perhaps reporters are adopting it because of their immersion in the prose of the sixteenth and seventeenth centuries, when the nonrestrictive that was common.
*I can’t resist saying again that you were mistaught if you were told that that may not be used to refer to people. That is a perfectly acceptable pronoun to identify groups of people, as in the example sentence, or a person whose name is not known.
