I’ve learned that “which” and “that” can look almost identical, yet choosing the wrong one can change the meaning or make a sentence sound awkward. If you’ve ever paused while writing and wondered which word fits, you’re definitely not alone.
In this guide, I’ll break down which vs. that with simple rules and real examples, so you can finally use both with confidence.
Quick Answer
Which and that can both be correct when they introduce a relative clause referring to a thing or animal. The main difference is that that is normally used for essential information, while which is required for extra, nonessential information set off by commas. In many varieties of English, especially American English, which can also introduce an essential clause.
The easiest rule is this: if the information is necessary to identify what you mean, that is a safe choice. If the information is additional and can be removed without changing the basic meaning, use which with commas.
Quick Comparison Table
| Word | Main Use | Example |
| that | Usually introduces an essential/defining relative clause | The book that I bought is new. |
| which | Introduces an extra/non-defining relative clause | The book, which I bought yesterday, is new. |
| which | Can also introduce an essential clause in many styles | The book which I bought is new. |
| that | Cannot normally introduce a non-defining clause | The book, that I bought yesterday, is new. ❌ |
The important point is that which is not simply “wrong” whenever that could be used. Both can work in many defining relative clauses, although some formal style guides prefer that in those situations.
What Does “That” Mean?
In the grammar pattern discussed here, that is a relative pronoun used to connect a noun with information that identifies or limits that noun.
For example:
The laptop that I ordered arrived today.
The words that I ordered tell us which laptop is being discussed. Without them, we only have:
The laptop arrived today.
That sentence is grammatically complete, but the original sentence gives us more specific information about the laptop.
That Usually Introduces Essential Information
An essential, or defining/restrictive, relative clause helps identify exactly which person or thing you mean.
For example:
The students that studied passed the test.
This can mean that the students who studied passed, while other students may not have passed.
Another example:
I need the shoes that are in the bedroom.
The clause tells us which shoes you mean.
That Can Refer to Things, Animals, and People
In everyday English, that can refer to things, animals, and sometimes people in defining relative clauses.
Examples:
The phone that I bought was expensive.
The dog that lives next door is very friendly.
She’s the person that helped me.
However, who is generally preferred for people, especially in careful or formal writing. Cambridge notes that that can replace who, whom, or which in defining clauses and is particularly common in informal speech.
What Does “Which” Mean?
Which has several grammatical uses, but in this article the important one is its use as a relative pronoun.
It commonly refers to things or animals and can introduce either a defining or non-defining relative clause. It can also refer to an entire preceding clause or idea.
For example:
The car which I bought is very reliable.
Here, which I bought identifies the car.
Now compare:
My car, which I bought last year, is very reliable.
Here, the information about buying the car last year is extra information.
Which Is Commonly Used for Extra Information
When which introduces a non-defining clause, commas normally separate the clause from the main sentence.
For example:
My laptop, which is three years old, still works well.
The basic sentence is:
My laptop still works well.
The information which is three years old adds detail but does not identify which laptop is meant.
Which vs. That: The Key Difference
The biggest difference is whether the relative clause is essential to identifying the noun or simply adds extra information.
Compare these two sentences:
The books that are on the desk belong to Sarah.
The books, which are on the desk, belong to Sarah.
The first sentence can suggest that only the books on the desk belong to Sarah. The relative clause limits which books we mean.
The second sentence treats the books as already identified and adds that they are on the desk.
This difference is often called the difference between a defining/restrictive clause and a non-defining/nonrestrictive clause.
Essential Information: Usually That
Use that when the relative clause is needed to identify the noun.
The dress that I ordered online finally arrived.
Which dress? The one I ordered online.
The employees that completed the training received certificates.
Which employees? The ones who completed the training.
Extra Information: Which
Use which when the clause simply adds information about an already identified noun.
My new dress, which arrived yesterday, fits perfectly.
The company car, which is parked outside, belongs to my manager.
In these examples, the commas show that the relative clause is additional information.
When Should You Use “That”?
In Everyday Conversation
That is very common when people identify a particular thing.
That’s the restaurant that I told you about.
This is the movie that everyone is talking about.
Where is the charger that I left here?
These sentences sound natural and straightforward.
In Formal Writing
That is also completely standard for defining relative clauses.
The policy that applies to all employees will be updated.
The evidence that supports the claim is limited.
Using that is often a clear choice when the clause is essential to the meaning.
After Superlatives
That is especially common after superlatives.
This is the best book that I have ever read.
She was the most talented student that I taught.
That was the funniest movie that we watched this year.
Cambridge specifically notes that that is often used after superlatives.
When the Clause Is Essential
A simple test is to remove the clause.
The phone that has a cracked screen is mine.
If you remove that has a cracked screen, you may lose the information needed to know which phone is yours.
That makes that a natural choice.
When Should You Use “Which”?
When the Information Is Extra
Use which for a non-defining clause.
My phone, which I bought in June, still works perfectly.
The phrase which I bought in June is additional information.
When Referring to a Whole Previous Idea
Which can also refer to an entire preceding statement rather than just one noun.
She missed the train, which caused a lot of problems.
Here, which refers to the situation described by the preceding clause.
Another example:
The store closed early, which surprised everyone.
The thing that surprised everyone was the fact that the store closed early.
Cambridge identifies this as a special use of which for referring to a whole sentence or clause.
When a Comma Is Required
If the clause is non-defining, which is the appropriate relative pronoun.
Our office, which is near the station, is easy to reach.
Not:
Our office, that is near the station, is easy to reach. ❌
Standard grammar guidance does not use that to introduce a non-defining relative clause.
When Should You Not Use “That”?
There are several important situations where that is not the right choice.
Do Not Use That in a Non-Defining Clause
Do not write:
My car, that is very old, needs repairs. ❌
Use:
My car, which is very old, needs repairs. ✅
The commas indicate that the information is extra.
Do Not Put a Preposition Directly Before That
You cannot normally use that immediately after a preposition in the way you can use which.
Incorrect:
The house in that I grew up. ❌
Better:
The house in which I grew up. ✅
Or, in natural everyday English:
The house that I grew up in. ✅
Cambridge notes that that cannot be used with a preposition immediately before it.
Do Not Use That Just Because You Are Talking About a Thing
A common oversimplification is:
People = who
Things = that
That is not the complete rule.
For things, both that and which can occur in defining relative clauses:
The computer that I bought is fast.
The computer which I bought is fast.
The second sentence is not automatically grammatically wrong. Usage varies by style and region.
When Should You Use “Which”?
For Non-Defining Relative Clauses
This is the clearest case:
The museum, which opened in 1950, attracts thousands of visitors.
For a Whole Previous Statement
Use which when commenting on what came before:
He forgot the meeting, which was unfortunate.
Here, which does not simply describe “the meeting.” It comments on the situation.
In Defining Clauses When Your Style Allows It
You can also see which without commas in defining clauses:
The laptop which I bought yesterday is already broken.
This usage is accepted in many forms of standard English. Merriam-Webster notes that both which and that can introduce restrictive clauses, although some traditional grammar advice strongly preferred that in this position.
Which or That in Common Sentence Situations?
Which or That for a Thing?
Both can be correct when the clause is defining.
The bag that I bought is black. ✅
The bag which I bought is black. ✅
If you want a simple, widely accepted choice for an essential clause, that is often the safest option.
Which or That After a Comma?
Use which.
My bag, which I bought yesterday, is black. ✅
Not:
My bag, that I bought yesterday, is black. ❌
The comma tells you that the information is non-essential.
Which or That After a Superlative?
That is very common.
This is the best restaurant that I’ve visited.
It was the hardest exam that I had ever taken.
Which may occur in some contexts, but that is a natural and widely used choice after superlatives.
Which or That After “Everything”?
That is commonly used.
Everything that I need is in this bag.
She remembered everything that we discussed.
The word that is especially natural in these defining structures.
Which or That After “The Only”?
That is often preferred.
She is the only person that I trust.
For people, who would usually be the more natural choice:
She is the only person who I trust.
But that is possible in defining relative clauses.
Which or That for a Whole Sentence?
Use which.
He arrived two hours late, which annoyed everyone.
Here, which refers to the situation expressed by the previous clause.
Using that in this pattern is not the normal standard construction.
Common Phrases and Patterns
The Thing That
The thing that matters most is honesty.
This pattern is extremely common because the clause identifies what “thing” is being discussed.
Everything That
Take everything that you need.
The clause specifies what belongs in the group of “everything.”
The Best That
This is the best advice that I’ve received.
The Only Thing That
The only thing that I want is a quiet evening.
Something That
I need something that will make this easier.
A Decision Which…
Which can occur in a defining clause:
We need a decision which everyone can support.
But that is also possible:
We need a decision that everyone can support.
In many contexts, that sounds more direct.
Common Mistakes
Mistake 1: Using “That” After a Comma
❌ My house, that is near the school, is easy to find.
✅ My house, which is near the school, is easy to find.
Why? The comma indicates that the clause is non-defining, so which is used.
Mistake 2: Thinking “Which” Is Always Wrong Without a Comma
❌ The book which I bought is automatically incorrect.
✅ The book which I bought is acceptable in many varieties and styles of standard English.
✅ The book that I bought is also correct.
Why? Defining clauses can use which as well as that in many forms of English.
Mistake 3: Using “That” After a Preposition
❌ The company for that I work.
✅ The company for which I work.
Or:
✅ The company that I work for.
Mistake 4: Using “That” to Refer to a Whole Previous Clause
❌ He forgot his passport, that caused a problem.
✅ He forgot his passport, which caused a problem.
Why? Here, which refers to the entire situation.
Mistake 5: Adding an Extra Pronoun
❌ The car that I bought it is expensive.
✅ The car that I bought is expensive.
The relative pronoun already fills the relevant grammatical role. You do not normally add another object pronoun such as it.
Mistake 6: Using “What” Instead of “That” or “Which”
❌ The book what I bought is interesting.
✅ The book that I bought is interesting.
✅ The book which I bought is interesting.
Standard relative-clause grammar does not normally use what in this pattern.
Mistake 7: Forgetting the Commas Around Extra Information
❌ My brother who lives in Canada is visiting us.
This may mean that you have more than one brother and you are identifying the brother who lives in Canada.
If you have one brother and the Canada detail is simply additional information:
✅ My brother, who lives in Canada, is visiting us.
The punctuation can therefore change the meaning.
Why Do People Get “Which” and “That” Wrong?
The confusion exists partly because the two words overlap.
Both which and that can introduce defining relative clauses referring to things. This means that learners sometimes encounter sentences that seem to follow different rules.
Traditional grammar advice also made the distinction sound stricter than everyday usage actually is. Some style traditions strongly favor that for defining clauses and which for non-defining clauses, while modern usage recognizes that defining which is common and accepted.
There is also a punctuation issue. Writers sometimes notice that which often appears with commas and conclude that which always requires commas. That is too simple because which can also occur in defining clauses.
Can Both Forms Be Correct?
Yes.
This is one of the most important points to understand.
Consider:
The phone that I bought is expensive.
The phone which I bought is expensive.
Both can be correct when the relative clause is defining.
The difference is partly one of style, convention, and regional preference rather than a simple “right versus wrong” rule. Merriam-Webster specifically recognizes which in restrictive clauses and notes that its use is established in modern English.
However, when the clause is non-defining, the distinction becomes much clearer:
My phone, which I bought yesterday, is expensive. ✅
My phone, that I bought yesterday, is expensive. ❌
So the practical rule is:
Defining clause: that or which may be possible.
Non-defining clause: which, not that.
British vs. American English
There is some variation between English varieties and between individual style guides.
In British English, which is commonly used in defining relative clauses:
The car which I bought was expensive.
That is also common:
The car that I bought was expensive.
In American English, many writers and editors traditionally prefer that for defining clauses and which for non-defining clauses, particularly in formal edited prose. However, defining which is still established and accepted in American English; it is not automatically an error.
This means you should not treat the distinction as a universal rule that every English speaker follows identically.
If you are writing formal English and want the clearest conservative choice, a useful approach is:
- Essential information → that
- Extra information → which, with commas
That rule will work well in a large number of situations.
A Simple Memory Trick
Think of that as a selector.
It helps select or identify one thing from a larger group.
The shirt that I ordered has arrived.
Which shirt? The one I ordered.
Think of which with commas as an aside.
My shirt, which is blue, has arrived.
You already know which shirt you mean. The color is simply extra information.
Remember It This Way
That = tells you which one.
Which + commas = tells you something extra.
This is not a complete description of every use of which, but it is an excellent practical rule for everyday writing.
Quick Decision Guide
Is the relative clause essential for identifying the noun?
→ That is a safe choice.
The file that I sent is missing.
Is the information extra and surrounded by commas?
→ Use which.
The file, which I sent yesterday, is missing.
Does “which” refer to the entire previous statement?
→ Use which.
She missed the deadline, which caused a problem.
Is there a preposition directly before the relative pronoun?
→ Use which, not that.
The company for which I work.
Or move the preposition to the end:
The company that I work for.
Mini Quiz
1. Choose the correct sentence.
A. The book that I bought is on the table.
B. The book, that I bought, is on the table.
Answer: A — The clause is defining, so that works naturally without commas.
2. Which sentence uses a non-defining clause correctly?
A. My car, that is very old, needs repairs.
B. My car, which is very old, needs repairs.
Answer: B — A non-defining relative clause uses which, not that.
3. Choose the natural sentence.
A. The best movie that I have seen was released last year.
B. The best movie which I have seen was released last year.
Answer: A — That is especially common after superlatives.
4. Which sentence is correct?
A. The company for that I work is large.
B. The company for which I work is large.
Answer: B — Which can follow a preposition directly.
5. Choose the correct word.
“She forgot her appointment, ___ caused a problem.”
A. that
B. which
Answer: B — Which refers to the whole preceding situation.
6. Which sentence is acceptable?
A. The laptop which I bought is fast.
B. The laptop that I bought is fast.
Answer: Both A and B — Both can be used in defining relative clauses in standard English, although some styles prefer that.
7. Which sentence is correct?
A. Everything that I need is here.
B. Everything which I need is here.
Answer: A — That is the natural and common choice in this defining structure.
Frequently Asked Questions
Is “which or that” a grammar question?
Yes. The choice between which and that often depends on the type of relative clause being used. The key distinction is whether the information is essential or additional.
Is “that” always used for things?
No. That can introduce defining relative clauses referring to people, animals, and things. However, who is generally preferred for people in many contexts.
Is “which” always used with commas?
No. Which is commonly used with commas when it introduces a non-defining clause, but it can also introduce defining clauses in standard English.
For example:
The book which I bought is excellent.
No commas are required because the clause can be defining.
Can I use “that” after a comma?
Normally, no, when the comma introduces a non-defining relative clause.
Use:
My car, which is old, still runs well.
Not:
My car, that is old, still runs well.
Which is more formal, “which” or “that”?
Neither word is simply “the formal one.” The choice depends on the grammatical structure. In defining clauses, some formal writing styles prefer that, while which is standard for non-defining clauses.
Is “the book which I bought” correct?
Yes. It can be correct when which I bought is a defining relative clause. The book that I bought is also correct.
Is “the book, which I bought” correct?
It can be correct if which I bought is intended as extra information.
For example:
The book, which I bought yesterday, is already on my desk.
The commas tell the reader that the information is additional.
Can “which” refer to an entire sentence?
Yes. This is an important use of which.
He missed the flight, which was unfortunate.
Here, which refers to the situation described in the previous clause, not simply to “the flight.”
Can I leave out “that” or “which”?
Sometimes.
In a defining relative clause, the relative pronoun can often be omitted when it functions as the object:
The book that I bought is excellent.
The book I bought is excellent.
But you cannot normally omit it when it is the subject:
The book that won the award is expensive. ✅
The book won the award is expensive. ❌
Cambridge notes that omission is possible in defining clauses when the relative pronoun is the object, but not when it is the subject or in non-defining clauses.
Which should I use in formal writing: “which” or “that”?
If the clause is essential, that is a clear and conservative choice.
If the clause is extra information, use which with commas.
This approach is simple, widely understood, and unlikely to cause confusion.
Final Takeaway
The choice between which and that is not simply a matter of one being correct and the other being wrong.
Both can introduce defining relative clauses in standard English:
The article that I read was helpful.
The article which I read was helpful.
But for a non-defining clause that adds extra information, use which:
The article, which I read yesterday, was helpful.
The simplest rule to remember is:
Use “that” for essential information, and use “which” for extra information set off by commas.
And remember one important exception to overly strict grammar rules: which can also be correct in defining clauses, especially depending on style and regional usage.
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