Many English learners feel confused about when to use “that” and when to use “which.” Both words introduce extra information about a noun, and sometimes they seem interchangeable. However, there is an important grammar rule—especially in formal writing—that helps you choose correctly.
Understanding the difference between “that” and “which” will improve your writing clarity, help you avoid common grammar mistakes, and make your English sound more natural and professional.
Why This Topic Matters
Using “that” and “which” correctly helps readers understand your meaning. In some cases, choosing the wrong word can change the meaning of your sentence.
The main difference depends on whether the information you are adding is:
- Essential (necessary) to the meaning of the sentence
- Nonessential (extra) information
This distinction is especially important in formal writing, academic English, and professional communication.
The Basic Rule
Use “that” for Essential Information (Restrictive Clauses)
Use “that” when the information is necessary to identify the noun. Without it, the sentence would not have the same meaning.
The book that is on the table is mine.
This sentence tells us which book. Without “that is on the table,” we don’t know which book the speaker means.
Use “which” for Extra Information (Nonrestrictive Clauses)
Use “which” when the information is extra and not essential. This clause is separated by commas.
My car, which is ten years old, still runs well.
Here, we already know which car: “my car.” The fact that it is ten years old is extra information.
Key Difference: Commas Matter
A simple way to remember the rule:
- That = No commas
- Which = Commas
If you see commas around the clause, “which” is usually correct.
Side-by-Side Comparison
| Feature | That | Which |
|---|---|---|
| Type of clause | Restrictive (essential) | Nonrestrictive (extra) |
| Commas? | No | Yes |
| Can remove clause? | No — meaning changes | Yes — main idea stays the same |
| Common in formal writing? | Yes | Yes |
Practical Examples
Let’s look at more real-life examples to make this clear.
Example 1: Essential Information
Students that study regularly perform better on exams.
Only students who study regularly perform better. The clause is necessary.
Example 2: Extra Information
The students, which were from Canada, arrived early.
This sentence is actually incorrect. We use “who” for people. The correct version is:
The students, who were from Canada, arrived early.
This reminds us that “that” and “which” are used for things, not people.
Example 3: Meaning Changes
The houses that are painted blue are expensive.
Only the blue houses are expensive.
The houses, which are painted blue, are expensive.
All the houses are blue, and they are expensive.
Notice how the meaning changes depending on the word and commas.
Example 4: Workplace Context
The report that you sent yesterday was very helpful.
This tells us which report.
The report, which you sent yesterday, was very helpful.
This suggests there is only one report, and the date is extra information.
Example 5: Everyday Conversation
I bought the phone that has the best camera.
You are identifying one phone from many.
I bought a new phone, which has an excellent camera.
You are adding extra information about the phone.
American vs. British English
In American English, the distinction between “that” and “which” is followed more strictly, especially in formal writing.
In British English, people sometimes use “which” even in restrictive clauses. However, in professional or academic writing, following the standard rule is recommended.
Can You Omit “That”?
In some sentences, “that” can be omitted in informal English.
The book (that) I borrowed was fascinating.
This is grammatically correct. However, in formal writing, keeping “that” is often clearer.
Note: You cannot omit “which” in nonrestrictive clauses.
Common Mistakes to Avoid
- Using “which” without commas in restrictive clauses (in formal American English)
- Using “that” after a comma
- Using “which” or “that” for people instead of “who”
- Forgetting that removing a restrictive clause changes meaning
Quick Summary: Key Takeaways
- Use “that” for essential information.
- Do not use commas with “that.”
- Use “which” for extra, nonessential information.
- Always use commas with “which” clauses.
- If removing the clause changes the meaning, use “that.”
- If removing the clause does not change the main meaning, use “which.”
Final Thoughts
So, is it correct to say “that” or “which”? The answer depends on the type of information you are adding to your sentence.
If the information is essential, use “that.” If it is extra information, use “which” with commas.
Mastering this small but important difference will make your English clearer, more precise, and more professional. With practice, choosing between “that” and “which” will become natural and automatic.