Which vs That: The Rule, the Test, and Real Examples

The difference between which and that comes down to one question: does the clause that follows the word change the meaning of the sentence, or just add extra information? If the meaning depends on the clause, use that with no commas. If the clause is just extra detail you could remove without losing the point, use which with commas. That's the rule. The hard part is recognizing which kind of clause you're writing.

This guide walks through the rule with a quick decision test, real examples drawn from consumer economics research, the history of Pittsburgh and Johnstown, and the difference between American and British usage. You'll also find common mistakes that show up in academic and professional writing, a "which hunt" proofreading technique, and answers to the questions most writers actually ask.

Quick Answer: Which vs That

Use that for restrictive clauses. A restrictive clause adds information the sentence needs to identify what's being discussed. No commas. Example: The house that flooded in 1936 was on Washington Street.

Use which for nonrestrictive clauses. A nonrestrictive clause adds extra information you could remove without changing the core meaning. Use commas. Example: The Johnstown Flood, which killed more than 2,200 people, struck in 1889.

The 30-second test. Read the sentence without the clause. If it still makes sense and means the same thing, use which with commas. If removing the clause changes the meaning or makes the sentence vague, use that with no commas.

The comma signal. If a comma feels right before the word, the word should be which. If a comma feels wrong, the word should be that.

Which vs That: At a Glance

The table below summarizes the core distinction. The detailed explanation follows.

Feature	That	Which
Type of clause	Restrictive (essential)	Nonrestrictive (nonessential)
Effect of removing the clause	Changes the sentence's meaning	Leaves the sentence's meaning intact
Commas	None	One before, one after (or one before plus end of sentence)
What the clause does	Identifies which one	Adds extra information about an already-identified noun
Refers to	Things, animals, sometimes places	Things, animals, sometimes places
Formal American English	Required for restrictive clauses	Required for nonrestrictive clauses
British English	Often used for both	Often used for both

Restrictive vs Nonrestrictive Clauses

The whole rule rests on understanding two kinds of clauses. Once you can tell them apart, the choice between which and that is automatic.

A restrictive clause identifies which one. It restricts the noun to a specific instance. Without the clause, the reader can't tell what the writer is talking about. Restrictive clauses use that and take no commas.

A nonrestrictive clause adds information about a noun the reader can already identify. The clause is extra detail, set off by commas. Remove it and the sentence still works. Nonrestrictive clauses use which and take commas.

Here's the same sentence written both ways with a Pittsburgh example:

Restrictive (that, no commas): The Pittsburgh neighborhood that lost the most jobs after the 1980s steel collapse was Homestead.

Nonrestrictive (which, with commas): Homestead, which lost the most jobs after the 1980s steel collapse, sits along the Monongahela River.

In the first sentence, the clause does the identifying. The reader needs it to know which neighborhood the sentence is about. In the second sentence, the noun is already identified (Homestead), so the clause just adds context that could be removed without changing the main point.

The Deletion Test

When you're not sure which word to use, run the deletion test. Read the sentence with the clause removed. If the sentence still says what you meant, the clause is nonrestrictive and you should use which with commas. If removing the clause changes the meaning or leaves the noun vague, the clause is restrictive and you should use that with no commas.

Here's the test applied to a consumer economics example:

Original sentence: Households that hold most of their wealth in housing are more vulnerable to local market shocks.

Without the clause: Households are more vulnerable to local market shocks.

Verdict: The meaning changed. The original sentence is about a specific kind of household, not all households. The clause is restrictive. Use that, no commas.

Now compare:

Original sentence: The 2022 Survey of Consumer Finances, which is conducted every three years, samples more than 4,000 US households.

Without the clause: The 2022 Survey of Consumer Finances samples more than 4,000 US households.

Verdict: The meaning is intact. The clause adds context about the survey's frequency, but the main fact about sample size still holds. The clause is nonrestrictive. Use which, with commas.

The Comma Test

A faster check works in the other direction. Read the sentence aloud. If you naturally pause before the word and would punctuate that pause with a comma, you want which. If you don't pause and a comma would feel intrusive, you want that.

Try it with a Johnstown history example:

Sentence A: The dam ___ failed above Johnstown in 1889 had been poorly maintained for years.

A pause before the missing word feels wrong. There were several dams in the region, and the sentence is identifying which one. Use that: The dam that failed above Johnstown in 1889 had been poorly maintained for years.

Sentence B: The South Fork Dam ___ failed above Johnstown in 1889 had been poorly maintained for years.

Now a pause feels natural. The dam is already identified by name, so the clause is just extra context. Use which: The South Fork Dam, which failed above Johnstown in 1889, had been poorly maintained for years.

The comma test isn't infallible, but it works for most sentences and pairs well with the deletion test as a double check.

Which vs That in Practice: Examples by Domain

The rule is the same across writing contexts, but the kinds of mistakes writers make depend on the kind of writing they do. Here are correct examples drawn from three domains where the distinction matters most.

Consumer Economics and Household Finance

Research writing in economics and finance leans heavily on relative clauses to define population subsets. Mixing up which and that can completely change what the writer is claiming.

Restrictive (that): Households that experienced unemployment during the 2008 recession reported lower financial risk tolerance a decade later.
Nonrestrictive (which): Financial risk tolerance, which Fisher and Yao (2017) measured using a four-item scale, varies systematically with gender and age.
Restrictive (that): Survey items that ask respondents to imagine hypothetical investment scenarios may produce different results from items that ask about real choices.
Nonrestrictive (which): The Survey of Consumer Finances, which the Federal Reserve has conducted since 1983, remains the gold standard for US household wealth data.

Pittsburgh and Western Pennsylvania History

Historical writing often introduces a place, person, or event and then adds detail about it. Whether the detail is essential or supplementary determines whether to use which or that.

Restrictive (that): The Pittsburgh mills that closed between 1979 and 1987 employed more than 75,000 workers at their peak.
Nonrestrictive (which): US Steel's Homestead Works, which closed in 1986, had been the largest steel mill in the world during its peak years.
Restrictive (that): The neighborhoods that lost the most population after deindustrialization were also the ones with the highest concentrations of mill jobs.
Nonrestrictive (which): The Mon Valley, which stretches southeast from Pittsburgh along the Monongahela River, lost more than 100,000 manufacturing jobs in the 1980s.

Johnstown and the 1889 Flood

The Johnstown Flood is one of the most documented events in American disaster history, and writing about it requires careful clause structure. The same sentence can shift meaning entirely depending on whether which or that is used.

Restrictive (that): The dam that failed on May 31, 1889 had been raised, lowered, and modified multiple times by the South Fork Fishing and Hunting Club.
Nonrestrictive (which): The South Fork Dam, which failed on May 31, 1889, had been modified by the South Fork Fishing and Hunting Club after they purchased the property.
Restrictive (that): The neighborhoods that sat closest to the Stonycreek and Little Conemaugh rivers suffered the heaviest damage.
Nonrestrictive (which): Cambria City, which lay directly in the path of the floodwaters, was almost completely destroyed.

American vs British Usage

The strict rule that that introduces restrictive clauses and which introduces nonrestrictive clauses is a feature of formal American English. British English treats the two words more flexibly, and many British writers use which for both kinds of clauses.

For academic publishing in the United States, follow the formal rule. Major American style guides, including the Chicago Manual of Style, the APA Publication Manual, and the AP Stylebook, all enforce the restrictive that versus nonrestrictive which distinction. Editors at US journals will flag mistakes routinely.

For writing aimed at British audiences, the rule is looser, but the comma rule still holds. Whether the relative pronoun is which or that, a nonrestrictive clause needs commas and a restrictive clause does not. The commas matter even when the word choice is flexible.

Common Mistakes with Which and That

The same errors show up over and over in academic papers, business writing, and student work. Knowing them in advance saves a round of revisions.

Using which for a restrictive clause without commas. A sentence like The mills which closed in 1986 employed thousands is a hybrid that breaks the rule both ways. If the clause is restrictive, change which to that. If the clause is nonrestrictive, add commas around it. Don't leave it half-formed.
Using that with commas. Restrictive clauses don't take commas. A sentence like The dam, that failed in 1889, had been poorly maintained is wrong. Either remove the commas (and keep that) or change the word to which (and keep the commas).
Forgetting the second comma. A nonrestrictive clause needs commas at both ends, not just one. The South Fork Dam which failed in 1889, had been modified by the club is missing the opening comma. Both commas or no commas.
Overusing that in formal writing. Some writers default to that in every sentence to be safe. The result reads as cluttered. When the clause is genuinely nonrestrictive, use which.
Using which or that for people. Both words refer to things, animals, or places, not to people. Use who for people. The economist who developed the framework, not the economist that developed the framework. Many style guides accept that for people in informal writing, but formal academic writing prefers who.
Restricting a unique noun. When the noun is already specific (a named person, place, or event), the clause that follows can only be nonrestrictive. The Johnstown Flood that killed 2,200 people is wrong because there was only one Johnstown Flood. The correct version is The Johnstown Flood, which killed 2,200 people.
Treating defining and nondefining clauses identically in translation. Writers translating from languages that don't grammatically distinguish restrictive and nonrestrictive clauses often produce sentences that mix the two. The fix is to think about meaning first, then choose the word and the punctuation together.

The "Which Hunt": A Proofreading Technique

Editors call the final pass for relative clause errors a "which hunt." The technique is simple and works on any document.

Search the document for every instance of which. Use the find function in your word processor.
For each one, ask whether the clause is essential. If you can remove the clause without changing the meaning, leave which alone but make sure commas surround the clause. If removing the clause changes the meaning, change which to that and remove the commas.
Search the document for every instance of that. Most uses will be correct, but watch for cases where the clause is genuinely nonrestrictive and which would read better.
Check the commas. Restrictive clauses (that) take no commas. Nonrestrictive clauses (which) take commas at both ends. Mismatches are the most common error.
Read suspicious sentences aloud. If you naturally pause before the word, the clause is probably nonrestrictive and the word should be which. If you don't pause, it's probably restrictive and the word should be that.

A which hunt takes about 20 minutes for a typical journal article and catches the great majority of relative clause errors. Run it as a separate pass, not folded into a general proofread, so you can give the patterns full attention.

Frequently Asked Questions

What is the difference between which and that?

The difference depends on the clause that follows the word. Use that to introduce a restrictive clause, which adds information essential to identifying the noun. Restrictive clauses take no commas. Use which to introduce a nonrestrictive clause, which adds extra information that could be removed without changing the sentence's core meaning. Nonrestrictive clauses take commas at both ends. The fastest test is to read the sentence without the clause. If the meaning's intact, use which with commas. If the meaning changes, use that with no commas.

When should I use that instead of which?

Use that when the clause that follows is restrictive, meaning the sentence needs the clause to identify what's being discussed. The dam that failed in 1889 above Johnstown was poorly maintained uses that because the clause identifies which dam. Restrictive clauses take no commas. If you can remove the clause without losing the sentence's meaning, the clause is nonrestrictive and the correct word is which, with commas at both ends.

Do I need a comma before which?

In formal American English, yes. Which introduces a nonrestrictive clause, and a nonrestrictive clause is set off by commas at both ends. The Survey of Consumer Finances, which the Federal Reserve has conducted since 1983, remains the gold standard for US household wealth data. There's a comma before which and another at the end of the clause. If a sentence uses which without commas in formal American writing, the writer most likely meant to use that instead.

Can I use which and that interchangeably?

Not in formal American English. The two words introduce different kinds of clauses. That introduces restrictive clauses, which carry information essential to the sentence. Which introduces nonrestrictive clauses, which add extra information. Swapping the two changes the meaning of the sentence. British English uses the two words more flexibly, and many British writers accept which for both kinds of clauses, but the comma rule still applies.

Can which refer to a person?

No. Which refers to things, animals, or places. For people, use who. The economist who developed the framework, not the economist which developed the framework. That can sometimes refer to people in informal writing, but formal academic writing prefers who for people in all clause types. The same comma rule applies: use who without commas for restrictive clauses and with commas for nonrestrictive clauses.

How do I know if a clause is restrictive or nonrestrictive?

Run the deletion test. Read the sentence without the clause. If the sentence still says what you meant, the clause is nonrestrictive and the correct word is which with commas. If the sentence loses important meaning or becomes vague, the clause is restrictive and the correct word is that with no commas. Restrictive clauses identify which one. Nonrestrictive clauses add extra information about a noun the reader can already identify.

Why do British writers often use which for restrictive clauses?

British English never adopted the strict restrictive that, nonrestrictive which rule that became standard in American formal writing during the twentieth century. The rule was popularized by the Fowler brothers in 1906 and reinforced by American style guides in subsequent decades. British style guides treat the two words as more interchangeable, though most still recommend which with commas for nonrestrictive clauses. For writing aimed at American audiences, follow the formal American rule. For writing aimed at British audiences, the comma rule matters more than the word choice.

What is the easiest way to remember the rule?

Two tests cover most cases. The deletion test asks what happens if you remove the clause. If the sentence still works, use which with commas. If it doesn't, use that with no commas. The comma test asks whether a pause before the word feels natural. If yes, use which. If no, use that. Used together, the two tests catch nearly every relative clause error in academic and professional writing.

Can a sentence have both which and that?

Yes. Sentences with multiple clauses can use both words for different purposes. The South Fork Dam, which failed in 1889, was modified by the club that owned the property uses which for a nonrestrictive clause about the dam and that for a restrictive clause identifying which club. The two words aren't in conflict. Each follows the same rule independently based on whether its clause is restrictive or nonrestrictive.

Do which and that mean the same thing in spoken English?

In casual spoken English, the distinction's often blurred. Speakers use which and that almost interchangeably and rely on tone of voice to signal whether a clause is essential or supplementary. In writing, especially formal writing, the distinction matters because the reader can't hear the speaker's pauses. The comma and the word choice carry the meaning that voice would convey in conversation. Following the rule in writing makes sentences clearer and prevents the kind of ambiguity that slows readers down.

Professional Editing for Clear, Correct Writing

The which versus that rule is one of dozens of small distinctions that separate polished writing from writing that sounds almost right. Most readers won't be able to name the rule. They'll just notice that one document reads cleanly and another doesn't. Reviewers, journal editors, and graders notice too, and the cumulative effect of small errors is what often turns a strong argument into a weak submission.

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This article was reviewed by the Editor World editorial team. Editor World, founded in 2010 by Patti Fisher, PhD, provides professional editing and proofreading services for graduate students, academics, and researchers worldwide. BBB A+ accredited since 2010 with 5.0/5 Google Reviews and 5.0/5 Facebook Reviews. More than 100 million words edited for over 8,000 clients in 65+ countries.