That vs. Which: The Comma Rule You’ll Never Forget

That and which cause confusion not because their meanings are complex, but because the difference depends on whether information is essential and—most importantly—on commas. In standard English usage, punctuation is not decoration here; it is the rule.

That vs. Which

When to Use That

That introduces a restrictive (essential) clause. This information is necessary to identify exactly which thing you are talking about.

The key signal:
A clause with that has no commas.

The Removal Test:
If you remove the clause and the sentence becomes unclear or changes meaning, you need that.

That Examples:

The movie that I watched last night is a big hit in Asia.
(Which movie? The one I watched.)
The car that has no brakes is extremely dangerous.
(Not every car—this specific one.)
The instructions that the researcher used were clear.

When to Use Which

Which introduces a non-restrictive (non-essential) clause. This information is extra—interesting, but not required to identify the noun.

The key signal:
A clause with which is always set off by commas in standard English.

The Removal Test:
If you remove the clause and the sentence still makes sense, use which.

Which Examples:

They entered the shop, which was a curious, ramshackle building.
My bike, which has ten speeds, is for sale.
This cake is made with ten eggs, which give it a rich taste.

A Clear Side-by-Side Comparison

That (essential):
The car that is speeding down the road is about to crash.
(You are identifying one specific car.)
Which (extra information):
My car, which is blue, has no brakes.
(You already know which car; “blue” is just extra detail.)

Fixing a Common Mistake

Original sentence:

My dog that is a Labrador loves to play fetch. ❌

Why it’s wrong:
Unless you have multiple dogs and are distinguishing one Labrador from the others, the breed is extra information.

Correct version:

My dog, which is a Labrador, loves to play fetch. ✅

Objects vs. People

This discussion applies mainly to things. When referring to people, English usually prefers who instead of that or which:

The student who answered the question was confident.

The rule that never fails:

That → essential information, no commas.
Which → extra information, commas required.

Final tip:
Before choosing that or which, try deleting the clause. If the sentence collapses, use that. If it survives, use which.

Last Updated on March 2, 2026

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Nhat Nhat

English Teacher | IELTS Instructor | English Learning Content Creator at ESLTeacher.org

Nhat Nhat is an English teacher and IELTS instructor specializing in grammar, academic writing, and English learning resources. He holds a Bachelor’s Degree in English Education from Hanoi National University of Education (HNUE), an IELTS 8.0 certificate, and TESOL / TEFL teaching certifications. With over 9 years of teaching experience, he creates practical lessons and educational content to help learners improve their English skills and exam performance.