Which, who, and that are the Huey, Lewey, and Dewey of pronouns. They’re relative.

Get it? Relative?

(Moms have dad jokes too.)

OK, ok, I'll get back on track.

Which, who, and that are indeed relative pronouns, which simply means they are pronouns that introduce dependent clauses.

Are your eyes glazing over already? I promise I will diagram no sentences. Stay with me.

Let's start with two quick definitions so that we're all on the same grammatical page. 

Relative pronoun: A pronoun that introduces a clause that modifies the noun it refers to (that, which, who, whom, what, whatever, whichever, whoever, whomever, whose)

Dependent clause (also known as a subordinate clause): A group of words patterned like a sentence, with a subject, verb, and possibly an object, that functions as noun, adjective, or adverb in the context of the larger sentence

So when you put these definitions together, which, who, and that are pronouns that introduce a new clause in the sentence in order to tell you more about the noun they refer to.

But knowing which of the three pronouns to use can be tricky because English likes to use these words in different ways. Which one you choose will affect your sentence's meaning.

Here’s a quick primer on what you need to know when using which, who, and that in American business writing using some realistic examples.

Note: Word mavens disagree about distinctions between relative pronouns. You can find varying opinions on "correct" usage depending on your source, your country of origin, and your level of nitpickiness. (If you're truly interested in this subject, I recommend checking out Amy Einsohn's masterful work, The Copyeditor's Handbook.)

When it comes to daily practices, the more esoteric copyediting rules are more complicated than they’re worth. I favor a "keep it simple" approach because it makes it easier to be consistent, which helps both the writer and the reader. The information given below describes the most prevalent usage of which, who, and that in American business writing.

That

That may be the most troublesome of the three relative pronouns because it is so flexible. In addition to being a relative pronoun, that can be a noun, an adjective, a conjunction, and an adverb—that's 5 different parts of speech for one 4-letter word. No wonder people get confused.

When that is used to set off a dependent clause, try to remember these two characteristics:

• That refers to things rather than people.
• That refers to information essential to the main point of the sentence, not just the nice-to-haves.

Let's look at an example:

Since 1985, our company has sponsored 22 public and 45 private partnerships that conduct drilling and development activities.

In this example, that begins a type of dependent clause known as a restrictive (or essential) clause, which is a fancy way of saying that what comes after the pronoun is critical to understanding the noun being referred to. The phrase that conduct drilling and development activities tells us what kind of partnerships we’re talking about. The company may have partnerships unrelated to drilling and development, but those aren’t the partnerships we care about right now.

Now let's take a look at that's fraternal twin, which.

Which

Like that, which is also an adjective as well as a pronoun and also always refers to “things” (objects, places, and entities). However, in American English, which is used for nonrestrictive, or nonessential, clauses. So a phrase that begins with which adds color to your understanding of the noun, but it doesn't define it.

Here's an example of which in action:

The developer will receive an oversight fee at a competitive rate, which according to management is $500,000 per well.

In this example, the word which correctly sets off the dependent clause because the information in that clause is useful but does not affect the sentence's purpose about the existence of a competitive fee for oversight.

Note the comma before which in the example above. The comma is an important tip-off here to signal that the which clause is not part of the main sentence and therefore is not essential.

So here's the big rule about commas with that and which:

If you use that in your relative clause, your information is essential to your main thought and you should not use a comma. If you use which to set off your relative clause, your information is still good but not absolutely critical to the main thought. You indicate this BOTH by using which and by setting the clause off with a comma.

To summarize: that—no comma; which—comma. Totally straightforward (once you get used to it.)

But I see which and that swapped on a regular basis. What’s going on?

It is not very hard to find which for that or that for which. This happens most frequently in British English, where the rules are just different. Again, the guidelines I stated above only apply for American English. If you are writing for a U.S. business audience, the distinction of that for essential information and which for non-essential information is pretty firm.

Who

Who is reserved for referring to a specific person or a group of people. The gotcha here is that who can be either restrictive or nonrestrictive; English doesn’t have another word to choose from.

This lack of a word to distinguish between essential and nonessential clauses can be problematic because the reader now must rely solely on the punctuation, or lack thereof, to signal the pronoun's meaning.

So your visual cue here for who clauses comes from your punctuation, just like the comma before which. If there is no comma before the who, the relative clause is essential to defining the subject. If a comma precedes the pronoun, the who clause is just providing something extra.

Let's look at a couple of examples that use who clauses.

Investors who pay a single wrap fee may acquire shares net of selling commissions.

(Restrictive use—only investors paying the fee may acquire units under the specified conditions.)

Investors, who otherwise have no voting rights, may on a majority vote remove the manager.

(Nonrestrictive use—the fact that the investors may only vote to remove the manager and not on any other issue is not the point of this sentence.)

What about whose? Does whose only refer to people?

Ahh, the relative possessive pronoun—a delightful exception to "things vs. people." Despite its preference for specificity and nuance, English has never developed a possessive pronoun just for things. So even if your subject is clearly not a person, we don’t say “which’s authority” or “that’s authority,” we say “whose authority.”

The positive side of whose as the only relative possessive pronoun is its simplicity. There's never any debate about what is correct; it's always whose. Enjoy this one beautiful respite from the general messiness of the English language. You won't get many others.

The Recap

Let's sum up what we've learned:

that—always for things or entities, does not automatically get a comma in front of it, always used to denote essential information

which—also always used for things or entities, always preceded by a comma when used as a relative pronoun

who—always used for people, preceded by a comma if the clause it introduces is not essential to the main point of the sentence

whose—used for every relative possessive situation 'cause there's no other choice