If you’re a speaker/writer of American English (AmE), these two relative pronouns may well strike terror into your heart. Which one to use? How to keep them straight in your head?
If you’re a speaker/writer of British English (BrE), you probably wonder what I’m on about in that first paragraph. (That is, unless you’re also versed in the quirks of AmE usage.)
In AmE, “which” is nonrestrictive. That means a clause that begins with “which” does not restrict the noun to which it refers. “Hand me the red cloak, which is behind the door.” In AmE this implies that there’s only one red cloak, and it happens to be behind the door. There could be other cloaks, but they’re not red (and presumably they’re not behind the door, either). Used in this way, “which” is always preceded by a comma. (That’s a rule. Not a guideline or a preference. A rule. Just do it.) The comma is a clue to the nonrestrictive nature of the clause. You don’t NEED that clause to make sense of the statement. It’s extra information. You’re being nice in providing it (oh, by the way, the thing I want is behind the door).
In AmE, “that” is restrictive. I’ll let you ponder that for a moment. ::waits:: “Hand me the red cloak that is behind the door.” The implication here is that there are several (perhaps even many) red cloaks, but you want the one that is behind the door (not the one that’s on the bed, or the one hanging in the closet). “That” restricts the noun to only one specific item in its class. Used in this way, “that” is NOT preceded by a comma. (See above parenthetical. Same deal here. Just do it.) You NEED that clause to make sense of the statement. It’s not extra information. It’s vital. (Give me the thing that’s behind the door. Not that other thing that’s somewhere else.)
It works the same way with plurals, by the way. “Ford has issued a recall of all Focuses built before 2008 that have ABS.” There are many more Focuses, total, but only those built before 2008 AND having ABS systems are affected. The noun is restricted to a subset of the class. (Please don’t bother telling me if there’s no such thing as a Focus built before ’08 with ABS. It doesn’t matter, factually speaking. It only matters as an example of restrictive pronoun usage.)
Garner’s Modern American Usage tells us that the use of “which” in AmE as a restrictive pronoun has reached Stage 4 (nearly a lost cause, sad to say).
British English observes a far less restrictive (heh) differentiation: “Which” and “that” may both be used in restrictive clauses. However, nonrestrictive usage demands “which,” the same as in AmE. If you require chapter and verse, I refer you to the New Oxford Style Manual (2012), 4.3.1, page 68, on the restrictive and nonrestrictive uses of the comma. (The pronouns are almost secondary to the section, which amuses me because I’m perverse that way.) I have yet to see a “that” appear where a “which” should be; I’m growing more used to seeing them used interchangeably (and properly so, per BrE rules) in the restrictive sense.
As an editor who works with writers from both sides of the pond (as they say), I’m used to seeing both the AmE and the BrE usages. My problem is remembering who I’m editing, and acting accordingly. If I’m editing a BrE writer, I leave whiches and thats alone. If I’m editing an AmE writer, I ask them what they prefer. Most of the time, they’ll tell me that they can never remember how it’s supposed to go and would I please make the corrections so they sound like they know what they’re doing.
I gladly do so. Readers will never know.