In Modern English Usage (1926), Fowler argues the case for limiting that to what he calls “defining clauses” and reserving which to introduce “non-defining clauses.”
Note: Fowler’s terms defining and non-defining correspond to restrictive and nonrestrictive.
Yet, here we are, more than 80 years later, and questions about when to use which and when to use that to introduce a clause are among those most commonly asked at this and other grammar sites.
The usual explanation begins like this:
THAT should be used to introduce a restrictive clause.
WHICH should be used to introduce a non-restrictive clause.
For starters, let’s look at the terms restrictive and nonrestrictive. In my own experience of learning grammar, I had a hard time trying to keep these terms straight. Perhaps I have too much imagination, but I kept thinking that the “restrictive clause” was the one that ought to have the commas, because, well, commas enclose things, don’t they? And enclosing something restricts it, no?
The editors of the Associated Press Stylebook must be aware of mindsets like mine because they reject the terms restrictive and nonrestrictive in favor of essential and nonessential:
These terms [essential and nonessential] are used in this book instead of restrictive…and nonrestrictive… to convey the distinction between the two in a more easily remembered manner. –AP Stylebook
These alternative terms certainly make it easier for me remember the distinction. The word essential means “absolutely necessary.” An “essential clause” is critical to the reader’s understanding of what the author has in mind; a “nonessential clause” is perhaps helpful or interesting, but can be omitted without altering the principal meaning of the sentence. Ergo, the nonessential clause is the one that gets the commas.
The nonessential clause is also the one that gets the which.
Mind you, using which to introduce an essential clause is not the unpardonable sin some readers–chiefly American–insist that it is. Writers of British English often use which to introduce an essential clause. Here are just two examples from sources committed to the dissemination of impeccable English:
We may link to external sites which give particular views of a person or organisation significant to a current news story… –BBC style manual.
The Royal Charter which governs our work sets out the objects for which we exist. –British Council website.
In the first example, particular sites are meant; in the second, a particular royal charter is being referred to. Both of these which clauses are restrictive/essential.
Banning the use of which to introduce essential clauses is a stylistic decision, not a grammatical necessity. Even the premier American style guide admits as much:
Although which can be substituted for that in a restrictive clause (a common practice in British English), many writers preserve the distinction between restrictive that (with no commas) and nonrestrictive which (with commas). –Chicago Manual of Style, 6.22.
That Chicago does not approve of using which to introduce an essential (restrictive) clause is made clear in the section titled “Good usage versus common usage”:
In polished American prose, that is used restrictively to narrow a category or identify a particular item being talked about: “any building that is taller must be outside the state”; which is used nonrestrictively—not to narrow a class or identify a particular item but to add something about an item already identified: “alongside the officer trotted a toy poodle, which is hardly a typical police dog.”–CMOS
That for essential clauses and which for nonessential clauses is without question the preferred American usage. And although which is still being used in British English to introduce some essential clauses, according to at least one British style guide, that is edging it out:
Restrictive [i.e., essential] clauses relating to things may begin with either that or which, although there is an increasing tendency for that to be preferred. – Penguin Writer’s Manual, p. 32.
Here are some sentences that reflect the preferred that/which usage:
The car that I want is out of my price range. (essential clause)
The car, which is only two years old, sold for $2,000. (nonessential clause)
The kitten that has white paws is the one I want. (essential clause)
The kitten, which was Jack’s favorite, never came back. (nonessential clause)