That Or. Which: When to Use Each Word

Choosing between that and which can be tricky, especially when you’re deciding whether a sentence needs a comma. Both words can introduce clauses that describe a noun, but in standard American English, they often signal an important difference.

Use that commonly for essential information that identifies a specific person or thing, and use which often for extra information that can be removed without changing the sentence’s main meaning. For example, “The book that has a red cover is mine” identifies one book among several. In contrast, “My book, which has a red cover, is on the desk” adds extra information about a book already identified.

Here’s the key test: does the clause tell readers exactly which person or thing you mean, or does it simply add detail? Essential clauses generally do not take commas, while nonessential clauses are set off by commas. Although this distinction is especially common in American English, British English allows which in some essential clauses, too. In this guide, you’ll learn the main rules, punctuation differences, practical examples, and common mistakes so you can write clearer, more accurate sentences.

That or Which – Main Difference

Use ‘that’ for restrictive clauses — essential information with no commas. Use ‘which’ for non-restrictive clauses — extra information set off by commas. Simple rule: Follow the rule, not your ear — the sentence decides which form is correct. Identify the word’s job in your sentence first — the rule then applies itself.

Two quick examples: “The book that I borrowed.” and “The book, which I borrowed,.”.

Memory trick: No commas — ‘that.’ Commas — ‘which.’ Think: ‘which’ needs a comma cushion.

One-line definitions:

• That: Use ‘that’ for restrictive clauses — essential information with no commas.
• Which: Use ‘which’ for non-restrictive clauses — extra information set off by commas.

The Origin of That or Which

Where the Rule Comes From

The rule behind this pair is older than most people guess. Both are ancient Germanic words. The restrictive-versus-non-restrictive rule was promoted by usage writers in the 1800s and adopted by American style guides.

British English is more relaxed about it. The rule is about commas as much as words: no commas, ‘that’; commas, ‘which.’

The rule was first written down by 18th-century grammarians who wanted English to behave like Latin. It stuck because it is genuinely useful.

How the Two Forms Diverged

The two forms then went their separate ways. Speech ignores the rule completely. Both words fit most sentences. The comma test is the only reliable guide, and many writers never learned it. Even editors debate edge cases.

Everyday speech began bending the rule almost immediately. Writing held the line — which is why the rule feels stricter on the page than in the air.

Why the Confusion Continues

Speech ignores this rule almost entirely, which keeps the confusion alive. Writing holds the line, but only where editors enforce it. The gap between spoken habit and written standard is where most errors are born.

Usage panels have debated this rule for decades, but the written standard has barely moved. That stability is good news: learn it once, and it serves for life.

British English vs American English Spelling

The rule itself is the same in American and British English, but strictness differs. American style guides enforce it firmly. British usage is more relaxed in speech, though formal writing on both sides follows the rule.

How Each Region Spells It

Grammar books in both regions teach the same rule. The difference is enforcement: American editors correct violations more aggressively, while British editors tolerate the informal form in casual contexts. Once you know the regional rule, the choice becomes automatic.

Examples in Context

Formal writing in both regions follows the rule; casual speech in both regions bends it. The examples below show the careful version — the one safe for exams and work. The context decides everything here, exactly as the table shows.

For example: “The day that changed.” and “Anything that helps.”.

Feature	That	Which
Use when	Use ‘that’ for restrictive clauses — essential information with no commas	Use ‘which’ for non-restrictive clauses — extra information set off by commas
Register	Standard	Standard
Example	The book that I borrowed.	The book, which I borrowed,.

The table distills the rule to three rows. When the sentence fits the left description, that is correct; otherwise, which.

Which Spelling Should You Use?

The rule is the same worldwide; only strictness varies. The four situations below show where you must obey it and where you can relax.

1. For American English

Follow the rule strictly. American style guides enforce this distinction, and American editors correct violations. In the US, the careful form is the expected form.

American exams test this distinction directly.

2. For British and Commonwealth English

British and Commonwealth English teach the same rule, though everyday speech bends it freely. In writing — especially exams and applications — follow it exactly as in America. British exams test it just as directly.

3. For Formal and Professional Writing

Formal writing is where the rule matters most. Essays, reports, and professional emails should all use the careful form. This is the version examiners and hiring managers expect.

Workplace writing follows the strict version without exception.

4. For a Global Audience

Use the careful form for global audiences. Learners and translators rely on the standard rule; the informal variant only confuses them. When in doubt, be strict.

Learners worldwide are taught the strict version first.

There vs. Their: How to Tell Them Apart

Common Mistakes with That or Which

The same six mix-ups appear in inboxes and essays everywhere. Study each pair — wrong versus right — and the pattern behind it will stick.

Mistake 1: “The book, that I borrowed, was long.”

Wrong: The book, that I borrowed, was long.

Correct: The book, which I borrowed, was long.

With commas, use ‘which.’

Mistake 2: “Cars which run well sell fast.”

Wrong: Cars which run well sell fast.

Correct: Cars that run well sell fast is the American rule.

No commas: ‘that.’

Mistake 3: “My car that is old.”

Wrong: My car that is old.

Correct: My car, which is old, needs work.

Extra info with commas: which.

Mistake 4: “The day which changed everything.”

Wrong: The day which changed everything.

Correct: The day that changed everything.

Essential info: that.

Mistake 5: “Anything which helps.”

Wrong: Anything which helps.

Correct: Anything that helps.

After ‘anything,’ use ‘that.’

Mistake 6: “Paris that is lovely.”

Wrong: Paris that is lovely.

Correct: Paris, which is lovely,.

Non-restrictive: which with commas.

Mistake 7: “The book which I borrowed.”

Wrong: The book which I borrowed.

Correct: The book that I borrowed.

The rule calls for ‘that’ in this sentence. ‘which’ breaks the rule.

Mistake 8: “The book, that I borrowed,.”

Wrong: The book, that I borrowed,.

Correct: The book, which I borrowed,.

The rule calls for ‘which’ in this sentence. ‘that’ breaks the rule.

That or Which in Everyday Examples

Real sentences beat abstract rules. Here is how that and which show up across daily life.

In Emails

• Cars that run well sell fast is the American rule.
• My car, which is old, needs work.
• The day that changed everything.

In News Articles

• Anything that helps.
• Paris, which is lovely,.

On Social Media

• The book that I borrowed.
• Cars that run well.
• The day that changed.

In Formal Writing

• Anything that helps.
• The book, which I borrowed,.

In Job Applications

• My car, which is old,.
• Paris, which is lovely,.
• The report, which was long,.

In Academic Writing

• The book, which I borrowed, was long.
• Cars that run well sell fast is the American rule.

That or Which – Google Trends & Usage Data

This pair generates evergreen search interest: the rule is short, the habit is strong, and the two keep colliding. Qualitatively, the picture never changes much.

What People Actually Search

Searches ask “that or which?” in huge numbers, usually with an example sentence attached. People want a ruling on their specific case, not a lecture. That intent — quick, practical, sentence-level — shapes every good answer.

The Speech-Writing Gap

In speech, almost everyone uses the informal form without noticing. In writing, the careful form is expected. Search data reflects the gap: people speak one way, then check the other way before publishing.

The rule lives in writing, not in talking.

How Usage Is Changing

The informal form keeps spreading in casual writing, following speech. But formal standards are not moving — style guides still prescribe the careful form. The safe strategy is unchanged: strict in writing, relaxed in talking.

Bottom line: the rule is stable and the searches are evergreen. Writers will keep checking their sentences against it — and careful writers will keep getting it right.

Then vs. Than: Time or Comparison?

That or Which: Side-by-Side Comparison

One last look at that and which, side by side. Find your situation in the left column, then read across.

Feature	That	Which
Meaning	Use ‘that’ for restrictive clauses — essential information with no commas	Use ‘which’ for non-restrictive clauses — extra information set off by commas
Status	Standard form	Standard form
Example	The book that I borrowed.	The book, which I borrowed,.

FAQs About That or Which

1. What is the difference between that and which?

Use ‘that’ for restrictive clauses — essential information with no commas. Use ‘which’ for non-restrictive clauses — extra information set off by commas. In short: they are two separate words with two separate jobs. Mixing them changes your meaning, so learn the distinction once and use it forever.

2. Which is correct: that or which?

It depends on the sentence. Use ‘that’ for restrictive clauses — essential information with no commas. Use ‘which’ for non-restrictive clauses — extra information set off by commas.

Apply the rule to your sentence and the answer is clear. Once learned, this distinction rarely needs relearning.

3. How can I remember the difference between that and which?

No commas — ‘that.’ Commas — ‘which.’ Think: ‘which’ needs a comma cushion. Say it to yourself each time you hesitate. After a few uses, the right choice becomes automatic — tricks work because they attach the rule to something vivid.

4. Why do people confuse that and which?

Speech ignores the rule completely. Both words fit most sentences. The comma test is the only reliable guide, and many writers never learned it.

Even editors debate edge cases. The confusion is so common that it has its own search traffic — you are far from alone. The fix is a single clear rule, which this guide gives you.

5. Can I use that and which interchangeably?

No. That and which do different jobs, and swapping them changes the meaning — sometimes to the exact opposite. Treat them as two separate words that happen to sound alike. File it with the small rules that make writing sharp.

6. What is the most common mistake with that and which?

Writing “The book, that I borrowed, was long.” when the correct form is “The book, which I borrowed, was long..” With commas, use ‘which.’ This single error accounts for a large share of all that/which mix-ups.

7. Will spellcheck catch the that/which mistake?

No — and that is exactly why this pair is dangerous. Both that and which are valid words, so no spellchecker will flag the swap. Only your knowledge of the difference catches it.

8. Which is more commonly used: that or which?

They serve different purposes, so frequency comparisons mislead. Each is common in its own context and rare outside it. What matters is not which is popular but which is right for your sentence. It is a small rule with a big payoff in clear writing.

9. Is which wrong or just informal?

It depends on the sentence. Where the rule calls for that, using which is an error in formal writing — though it may pass unnoticed in casual speech. In exams and professional writing, treat the rule as strict.

10. Do native speakers follow this rule?

In careful writing, yes. In everyday speech, often no — the informal form is everywhere. That split is exactly why the rule is worth learning: writing is where it counts.

11. What happens if I mix them up?

Usually nothing terrible — readers understand you. But in exams, applications, and professional writing, the error signals carelessness. The fix takes seconds; the impression lasts.

12. Is the that/which rule taught in schools?

Yes — it is a standard exam topic in both American and British schools. Tests love it precisely because speech ignores it. Students who learn the written rule gain easy marks. File it with the small rules that make writing sharp.

Conclusion: That or Which

The that versus which question looks small, but it sits at the heart of clear writing. Use ‘that’ for restrictive clauses — essential information with no commas. Use ‘which’ for non-restrictive clauses — extra information set off by commas. One test — meaning, region, or grammar, depending on the pair — settles it every time.

Keep the memory trick close: No commas — ‘that.’ Commas — ‘which.’ Think: ‘which’ needs a comma cushion. Tricks like this survive because they turn a rule into a reflex. Use it deliberately for a week, and the right form will start choosing itself. Writers who master a few such tricks rarely second-guess themselves again.

Final advice: slow down on exactly these two words. They are the kind of pair that spellcheckers ignore and readers notice. A two-second check now saves an embarrassing correction later — and every correct use makes the next one easier.

If you remember nothing else, remember this: Follow the rule, not your ear — the sentence decides which form is correct. That single sentence covers nearly every real-world case. And the next time that and which make you pause mid-sentence, you will know exactly what to do.