It is a lovely summer afternoon, as I write; the sky is blue, the clouds are puffy, and a gentle breeze is blowing.
And here I am at my desk, doing a little summer grammar review. I want to make sure I'm clear on the use of which and that in complex sentences.
Both are relative pronouns; they introduce dependent, or subordinate, clauses. They are "relative" – they "establish a relation between" subordinate and main clauses. That introduces essential, or restrictive, clauses. Which introduces nonessential, or nonrestrictive, clauses: material that's not necessarily superfluous, but just not the main point.
For instance: "The house that Peter has bought was once the home of a famous professor." "That Peter has bought" is essential; without it you don't really have much of a sentence.
By contrast: "Peter's house, which he's owned for several years, was designed by a famous architect." The "which" clause has useful but not essential information.
Mignon Fogarty, aka Grammar Girl, says which/that questions are among the most frequent ones she gets, and explains: "Here's the deal: some people will argue that the rules are more complex and flexible than this, but I like to make things as simple as possible, so I say that you use that before a restrictive clause and which before everything else."
Here's her mnemonic: "If you think of the Wicked Witch (Which) of the West from 'The Wizard of Oz,' you know it's okay to throw her out. You won't change the meaning of the sentence without the which phrase. So, you can throw out the which (or witch) clause, commas and all."
The Chicago Manual of Style takes much the same line, albeit a bit less breezily: "In polished American prose, that is used restrictively to narrow a category or identify a particular item being talked about…; which is used nonrestrictively – not to narrow a class or identify a particular item but to add something about an item already identified."
But Grammar Girl is right: Some people will argue that the rules are more complex and flexible than that.
For one thing, with prepositions, you have to use which: "The house in which he has lived for 20 years...."
Also, some authorities call for which with an essential clause if you already have an essential clause with that: "He said Monday that the part of the army which suffered severe casualties needs reinforcement."
That sentence, from The Associated Press Stylebook, appears in a number of style guides.
That so many guides use the same example tells me that this nuance is not exactly graven on the hearts of the people. One Air Force guide that cites it adds, "Follow this rule even if your word-processor's spelling and grammar check function prompts you to change your use of who, that or which in your writing."
A further wrinkle here is that British usage varies somewhat from American.
In its style guide, The Economist, which generally takes a no-nonsense view of things, enunciates the same principle as Grammar Girl, but then uncharacteristically fudges: "Americans tend to be fussy about making a distinction between which and that. Good writers of British English are less fastidious. ("We have left undone those things which we ought to have done.")
The quotation is from the "Book of Common Prayer," which was published in 1928 – before people let their grammar-checkers push them around.