That is a handy and versatile word. It can be an adjective:

That puppy followed me home.

Or an adverb:

Are you that sure of yourself?

Or a pronoun, standing in for a person, place, thing, event, or anything else that is clear from context:

That is the way we’ve always done it.

Or a conjunction:

She insisted that we show up on time.

Or a relative pronoun, which is sort of a cross between a pronoun and a conjunction:

Any map that shows my road as two-way needs to be updated.

In the grammar and usage section of the Chicago Manual of Style (CMS), Bryan Garner puts it more elegantly: “A relative pronoun is one that introduces a dependent (or relative) clause and relates it to the independent clause. Relative pronouns in common use are who, which, what, and that” (CMS 16th ed., section 5.54).

Here’s where things get interesting, maybe a little confusing, and sometimes even contentious. These “dependent (or relative) clauses” can be either restrictive or nonrestrictive, or, as they’re sometimes called, essential or nonessential. Borrowing from CMS again: “A relative cause is said to be restrictive if it provides information that is essential to the meaning of the sentence. . . . A relative clause is said to be nonrestrictive if it could be omitted without obscuring the identity of the noun to which it refers or otherwise changing the meaning of the rest of the sentence” (CMS 16, section 6.22).

Here’s a restrictive clause:

The novel that we’re reading this month can be found in the library.

And here’s a nonrestrictive one:

Margaret Atwood’s The Handmaid’s Tale, which we’re reading for class, was published in 1985.

Here’s where the contentiousness comes in. Have you in your travels come across the “which/that distinction”? I don’t believe I was aware of it before I got my first editing job, in the publications office of a big nonprofit in Washington, D.C. My mentor was a crackerjack editor with decades of experience in New York publishing. Thanks to her I’ve been able to more or less support myself as an editor all these years. She was a stickler for correct usage, and as her apprentice I internalized most of her thou shalts and thou shalt nots.

High among the thou shalts was Thou shalt use only “that” for restrictive clauses and only “which” for nonrestrictive clauses, and “which” must be preceded by a comma.

This is the which/that distinction, and like many another novice editor I became a zealous enforcer of it. Worse, I became a tad smug about it. As I like to say, “everyone’s the hero of their own story,” and for copyeditors, whose brains are crammed with rules and guidelines, this sometimes leads to a conviction that it’s our esoteric knowledge that stands between us and the collapse of civilization, or at least the English language.

In other words, I looked down my snoot at any writer who used “which” for restrictive clauses.

Until I noticed, before too many years had passed, that writers of British English (BrE) regularly used “which” for restrictive clauses without their prose collapsing into a muddle.

And that speakers of American English (AmE) often don’t make the distinction in conversation.

Hmmm. By this point, the which/that distinction was so ingrained that I applied it without thinking in my own writing and in editing as well. This stands me in good stead in U.S. trade publishing, where which/that is still a thou shalt in many quarters.  Nearly all of the AmE writers whose work I edit apply it automatically, so I don’t have to change anything.

With BrE writers, at first I’d diligently change all the restrictive whiches to thats, but then I started getting uneasy. The which/that distinction is a convention, not a rule. I wasn’t improving the prose in any way by enforcing it. Most important, by diligently enforcing a distinction that didn’t need to be enforced, I was pretty sure I was missing more important stuff. Like many other editors and proofreaders I learned early on that the mistakes I missed usually came in close proximity to the ones I caught. It’s as if the editorial brain takes a self-congratulatory pause after each good catch, and in that moment an obvious error can slip through.

So I stopped automatically changing all those whiches to thats. I’d note on my style sheet “which OK for restrictive clauses” so the proofreader wouldn’t flip out and think the copyeditor was asleep at the keyboard. So far the language hasn’t collapsed and my publisher clients haven’t dumped me, but it still feels a little daring so I look over my shoulder a lot to see who’s watching.

Garner notes that the restrictive that is used “in polished American prose,” but that “in British English, writers and editors seldom observe the distinction between the two words.”

Says the usage note in Merriam-Webster’s:

Although some handbooks say otherwise, that and which are both regularly used to introduce restrictive clauses in edited prose. Which is also used to introduce nonrestrictive clauses. That was formerly used to introduce nonrestrictive clauses; such use is virtually nonexistent in present-day edited prose, though it may occasionally be found in poetry.

In its much lengthier usage note, American Heritage comes round to more or less the same conclusion: “But this [restrictive] use of which with restrictive clauses is very common, even in edited prose.” The whole note is an excellent introduction to which/that and restrictive/nonrestrictive. Check it out.