Which vs that: when to use each in business writing

Picture an email to a client: "Please pay the invoice, which is overdue." The client has three open invoices with you. Which one do you mean? They can't tell, and neither can the person in their accounts office who has to approve the payment.

That one comma is the whole which vs that problem in miniature. We'll give you a ten-second test for it, show what the main US style guides really say, and fix a made-up client email line by line. You won't need a grammar lecture for any of it.

That points to one thing; which adds a side note

Both words start a clause that describes a noun, and that's why they get mixed up. The difference is what the clause does. A restrictive clause is a clause that narrows the noun down to the one you mean, and it takes that. A nonrestrictive clause is a side note about a noun the reader can already identify, and it takes which plus commas. Purdue OWL puts the choice the same way: it "depends on whether the clause it introduces is restrictive or non-restrictive."

Here's how that looks in a real business.

Example: Osprey Ledger Bookkeeping is a made-up two-person bookkeeping firm in Fargo, North Dakota. Its owner writes "Our office, which is on Broadway, opens at 8 a.m." The firm has one office, so the street name is a side note. Now imagine it grows to three offices. "The office that is on Broadway opens at 8 a.m." tells a client which of the three opens early, and the other two may not.

Purdue OWL's own pair shows the stakes in seven words. "They have two sons who are doctors" means there are more sons. Add one comma, "They have two sons, who are doctors," and the family has only two. Same words. Different family.

Cover the clause and reread: the delete test

That comma difference gives you the fastest way to decide when to use which vs that. Cover the clause with your thumb, reread what's left, and ask whether the reader still knows exactly which thing you mean.

1. Find the clause that starts with which or that and ends where the sentence picks up again.
2. Cover it, or cut it from a copy of the sentence.
3. Reread. If the sentence still points to the same thing and says something true, use which with commas. If it now points to the wrong thing, or to everything, use that with no commas.

Microsoft’s Writing Style Guide gives the same test in plain words: "If you can omit the clause and the sentence still makes sense, use which, and put a comma before it."

Try it on "Please pay the invoice that is overdue." Cut the clause and you get "Please pay the invoice," which leaves the client guessing. So that stays.

The test also fixes our opening email. If the client has one open invoice, write "Please pay invoice 1042, which is overdue." The number identifies it, and the overdue part becomes a side note, so it gets which and a comma. The Chicago Manual of Style's own answer on this question works the same way: if you remove the clause and the sentence still makes sense, it's nonrestrictive.

Do you need a comma before which or that?

So which goes with commas and that goes without. That's the whole which vs that comma rule, and it matters more than people think because a comma can change who a sentence covers.

Read these two lines from a refund policy:

"We refund orders that ship late."

"We refund orders, which ship late."

The first promises a refund on late orders only. The second says every order ships late and every order gets a refund.

Nobody means the second one, but a stray comma can put it on your website.

Use two commas, not one. A nonrestrictive clause in the middle of a sentence needs a comma before it and after it, as in "Our refund policy, which changed in March, now allows 30 days."

Purdue OWL says it outright: "commas are used to separate the non-restrictive clauses from the rest of the sentence." A restrictive clause takes none, and that includes the spot right before that.

One honest limit here. We're talking about how readers parse a sentence, not about how a court would read your terms. If a refund policy, contract or warranty matters to your business, have a lawyer read the final wording.

What Chicago, AP and Microsoft actually tell US writers

Those comma rules are where the US style guides agree. Where they differ is on which without commas, and that's the part most grammar posts skip.

Chicago's editors put it plainly in their Q&A: "Some people use “which” restrictively, which is more or less okay (and popular among writers of British English) as long as no commas are involved." That's a long way from "always wrong."

Microsoft is stricter, as you'd expect from a guide written for product and help text read worldwide. Its rule starts: "Use that at the beginning of a clause that’s necessary for the sentence to make sense. Don’t put a comma before that." The AP Stylebook isn't free to read, so we haven't quoted it. A 2011 PR Daily guide quotes AP's definitions of essential and nonessential clauses and pairs them the usual way: that for essential, which for nonessential.

Even the federal government doesn't hold the line. The Office of the Federal Register's Document Drafting Handbook tells agencies how to write rules, and its own text says "Use asterisks to represent text which is not changed." No comma, restrictive which, and the meaning is clear.

Our take: in business writing, use that for restrictive clauses every time. It's never wrong in US English, and it removes one thing a reader could misread. But don't spend a review round "fixing" a correct which without commas in someone else's draft. Spend it on the commas.

The right word depends on facts outside the sentence

If the commas matter that much, it's tempting to hand the job to software. But the right choice turns on a fact that isn't in the sentence at all: how many offices, invoices or portals you actually have.

Take a line we wrote for this guide: "The invoice, that we sent on May 3, is still unpaid." It's wrong under every guide above, because a that clause never sits between commas. But it has two correct fixes, and they say different things. If the client has one open invoice, write "The invoice, which we sent on May 3, is still unpaid." If they have several, drop the commas: "The invoice that we sent on May 3 is still unpaid."

Both fixes are good grammar, so the words alone can't tell a reader, or a grammar checker, which one is true. That depends on whether your company sent one invoice or five, and has one office or three.

Only you know that.

Where software does help is finding the sentences worth checking. Which-clauses pile up in long sentences, so paste your draft into our free readability checker. It highlights long and passive sentences, and those are the ones to read with the delete test.

It's the same reason AI detectors disagree with each other: a tool scores patterns, not what you meant.

Three places which is right without a comma

Not every which needs commas, so don't go hunting for every one of them. Three uses are correct in every US guide we read.

First, after a preposition. "The account to which we send statements" is restrictive, and that can't follow a preposition. Purdue OWL says which is used instead of that after a preposition and lists "in which," "for which" and similar pairs. In everyday writing you'd usually say "the account we send statements to," which is also fine.

Second, when which asks a question or picks between options. "Which plan fits a five-person team?" has no relative clause at all, so the rule doesn't apply.

Third, when a comma-plus-which clause comments on the whole idea before it: "The client paid early, which saved us a reminder." Here the comma is required, and that won't work.

One more habit to keep: don't strip out every that to save words. Microsoft's guide says to "Include that even if the sentence is clear without it," because it clarifies the sentence structure. "We confirmed the payment arrived Friday" can make a reader stop: did you confirm the payment, or that it arrived? "We confirmed that the payment arrived Friday" settles it.

Fix one email in five minutes

Rules stick when you use them on your own words. Here's the before and after on a client email from our made-up Fargo firm.

Before: "Hi Dana, the report which you asked for is attached. Our new portal, that launched Monday lets you download statements which are older than 90 days."

After: "Hi Dana, the report that you asked for is attached. Our new portal, which launched Monday, lets you download statements that are older than 90 days."

Three changes. The report is the one Dana asked for, so that. The firm has one portal, so which, with the missing closing comma added. And only older statements download there, so that. It takes about as long to fix as it does to read.

Run the same pass on the text people read most: your website's service pages, your policies, your media kit and the calls to action in your Facebook posts, where one misread line can cost you the click you paid for. Email signatures and replies from your business email address count too.

Clear sentences pay off in search as well. Our SEO content writing tips come back to writing for the reader first, and our guide to getting cited by AI answers shows why self-contained sentences get quoted. For more on writing that sells, see our copywriting basics for beginners, and when the post is ready, our plan for promoting a business blog post.

Government writers are held to this too. Agencies follow federal plain language guidance, and the Office of the Federal Register's page on readable regulations sends rule writers there. Your customers deserve the same clarity.

Want someone else to do the editing? Our content marketing team writes and edits web pages and blog posts for small businesses. It's a paid service of ours, so weigh that when you read this suggestion.

How we made this guide: our team read each style source in full, last checked October 24, 2026. They were the Chicago Manual of Style's free Q&A, Microsoft's Writing Style Guide, three Purdue OWL pages and the Federal Register's Document Drafting Handbook. The AP Stylebook is paywalled, so AP's wording comes from a PR Daily article and is labeled that way. We wrote every business example ourselves and ran each one through the delete test before it went in. Osprey Ledger Bookkeeping is made up. Every fact links to its source where it appears. Eram N. reads and approves the rewrite before it goes live.

First published May 2025. Rewritten in full October 24, 2026: new text from US style guides, business examples, a comma section on policies, a worked invoice example and a before-and-after email. The old copied sections and broken examples are gone.

Sources

• The Chicago Manual of Style Online: Q&A, Which vs. That
• Microsoft Writing Style Guide: that vs. which
• Purdue OWL: That vs. Which
• Purdue OWL: Relative Pronouns, Introduction and General Usage in Defining Clauses
• Purdue OWL: Defining vs. Non-Defining Clauses and ESL Tips
• Office of the Federal Register: Document Drafting Handbook (PDF)
• Office of the Federal Register: Document Drafting Handbook page
• Office of the Federal Register: Making Regulations Readable
• Digital.gov: Plain language guide series
• PR Daily: That or which? A guide for writers (January 25, 2011)