Do you use a comma before a "who," "which," or "that" clause?
Editing grammar for accuracy and punctuation for clarity
The Short Story
We sometimes use who, which, and that to introduce incomplete thoughts called relative clauses, which provide additional information about someone or something earlier in the sentence (whatever previously mentioned person, place, thing, or notion who or which or that refers to). If it’s a person, use who (or a variaton thereof). If the relative clause is absolutely necessary to pinpoint a place, thing, or idea, it’s best to use that without a comma. If the relative clause conveys optional, nice-to-know details, always use which with a comma (or alternative) beforehand (and some kind of punctuation afteward, too).
The Real(ist) Story
Who, which, and that are words capable of wearing a few different hats. That can serve as a pronoun, an adjective, an adverb, or a conjunction, for starters. Here we’re concerned with these words’ use as (relative) pronouns introducing (relative) clauses. Relative clauses have subjects and verbs, like any good clause, but they’re dependent (subordinate), unable to stand on their own independently as complete thoughts—if only because of the pronoun itself.
A relative clause, which conveys information about some person or thing the relative pronoun refers to, may be restrictive or nonrestrictive. Counterintuitively, the nonrestrictive version gets fenced in by a comma or two, while the restrictive version doesn’t. That’s because the “restrictive” bit has to do with meanings, not punctuation. The extra information in a restrictive clause narrows (restricts), or clarifies, our understanding of the person or thing the clause refers to. Because the clause is essential to a clear understanding of who or what we’re talking about in the first place, we keep it joined at the hip to the previous part of the sentence—without the divider of an extra comma.
The information in a nonrestrictive clause elaborates in a nonessential way; it could be plucked out and whipped into a separate sentence without changing the “what” or “who” of the original sentence in any critical way. Because the clause provides extra, elaborating detail rather than essential, clarifying detail, we bracket it off with a comma at the beginning and another comma, or end punctuation, at the finish.
I speak of commas here, but substitute em dashes or parentheses if the situation calls for it. To tell you true, I find it helps to think of the bracketing commas (or comma and end punctuation) as parentheses. That way, the nonrestrictive clause seems more like an aside or afterthought—in other words, optional. (When you read it aloud, you’re also more likely to pause before a nonrestrictive clause.)
All of these grammatical underpinnings help explain my advice on commas before who relative clauses, which relative clauses, and that relative clauses. It’s less a matter of which words get commas and more a matter of which words can be used in comma (nonrestrictive, nonessential info) situations or no-comma (restrictive, essential info) situations.
Who (or whom or whose, as the case may be) means the relative clause provides information about a person. Never use that to refer to a person—have a little respect, y’all. You’ll have to make the judgment whether it’s a nonrestrictive or restrictive clause to know whether or not to use a comma with who.
Which almost always means a nonrestrictive clause, so it usually gets a prefatory comma. Double-check, because every so often, someone will try to be clever and sneak which into a restrictive clause. Doing so may not be wrong, but I’d argue it’s less right—less natural-sounding for most of us. Besides, where there’s a choice, I’m partial to using distinct words for distinct purposes. Call me precise.
That leaves, err, that. If that begins a relative clause, don’t use a comma. That is only used in restrictive situations; since it’s a specialist in this regard, I find it’s the clearer choice.
If you take nothing else from this post, stick these guidelines in your editorial toolbelt: Use who to refer to people. Otherwise, if you pause before you read the clause aloud, or if that would sound funny, use a comma and which. And don’t use a comma before that.
Story Time
Below are a few examples pulled from my Dustsong novella.
Who clause (nonrestrictive)
The barbarian glanced at the other woman present, who hovered close by the stone-beast.
Here, Egwae was the only other woman (loosely speaking) present besides Kaelii (the barbarian). The detail that Egwae “hovered close by the stone beast” was therefore additional information, not necessary to distinguish betwixt multiple candidates for “the other woman present.” Nonessential means nonrestrictive, which in turn means a prefatory comma. And Egwae was decidedly a who.
Who clause (restrictive)
“But I don’t recall offending anybody, let alone the storm-tamer who bested Livyat.”
Here Len referred to the Wright (a who) and his victory over the primordial chaos serpent, Livyat. It may seem like we’ve stumbled into a grey area with this example. Unless the Vaporous Realms story-world exists in your head the way it does in mine, you might be unsure whether it contained “storm-tamers” other than the Wright. So, “to comma” or “not to comma”?
Note that Len said the storm-tamer instead of using Storm-Tamer as an epithet. Potentially, Len thought his brother Sceg, to whom he was speaking, wouldn’t immediately know who the storm-tamer was without the clarifier who bested Livyat. But an alternative, more likely explanation of why it’s a restrictive clause is that in context, Len used the entire phrase the storm-tamer who bested Livyat (not merely the storm-tamer) as a way of naming the Wright. You wouldn’t put a comma in the middle of a name.
Which clause (nonrestrictive)
He paused for a respite under the diminutive pear trees, which had taken up residence on a flat, especially green tract of mountainside.
The diminutive pear trees were the only pear trees around, so the additional details about their location weren’t necessary to identify the trees in question. If a relative clause isn’t for essential clarity, it’s nonrestrictive, hence the bracketing commas and—given that pear trees aren’t people—the use of which.
Note that it wouldn’t sound wrong if you were to remove the comma and replace which with that. However, it would change the meaning of the sentence subtly, suggesting the presence (or possibility, at least) of more pear trees in the vicinity, beyond those at the specific spot described.
That clause (restrictive)
Len’s cheeks warmed in a way that had naught to do with the wastes. He hadn’t a notion what was so funny.
Len’s cheeks might have warmed in any number of ways, so it’s necessary to narrow the possibilities with a restrictive clause. The way in question was one having nothing to do with the desert sun—which is an indirect clarification, but it counts. In context, the clause clearly suggests Len was embarrassed. Therefore, we omit the comma. Though Len was a person, his cheeks were not, so that (not who) is appropriate.
Godspeed and happy rewriting!