Grammar B1–B2 6 min read Updated 10 June 2026

Quick answer: Use that for restrictive clauses — they limit which thing you mean and take no commas: “The book that I need is on the shelf.” Use which for non-restrictive clauses — they add extra information set off by commas: “The book, which I bought yesterday, is fascinating.” A simple test: if removing the clause changes the meaning, use that; if removing it leaves the meaning intact, use which (with commas).

Comparison Table

WordMeaningPart of SpeechExample
thatintroduces a restrictive (defining) relative clause; limits or identifies which thing is meantrelative pronoun / conjunctionThe car that I borrowed broke down.
whichintroduces a non-restrictive (non-defining) relative clause; adds extra information about something already identifiedrelative pronounMy car, which I bought last year, broke down.

Using That (Restrictive Relative Clauses)

That introduces a restrictive (also called defining) relative clause. This type of clause is essential to the sentence — it tells you exactly which person, place, or thing is being referred to. Without it, the meaning of the sentence changes or becomes unclear. Restrictive clauses introduced by that do not use commas.

The film that won the Oscar was directed by a British filmmaker.

She returned the book that she had borrowed from the library.

I want a laptop that has a long battery life.

The emails that you sent yesterday were very helpful.

In each sentence above, removing the that-clause would leave the meaning incomplete or overly vague. The clause is doing essential identifying work.

When That Can Be Omitted

In restrictive clauses where that is the object (not the subject) of the clause, it can be omitted entirely in informal speech and writing:

  • The book [that] I need is on the shelf. — that is optional here (object of need)
  • The film [that] she recommended was excellent. — again optional
  • The man that called you left a message. — here that is the subject; omitting it is less natural

Using Which (Non-Restrictive Relative Clauses)

Which introduces a non-restrictive (also called non-defining) relative clause. This type of clause adds bonus information about something that is already fully identified. The information is interesting or relevant, but the sentence makes complete sense without it. Non-restrictive clauses are always set off by commas — one before and, if the clause falls mid-sentence, one after.

The Eiffel Tower, which was completed in 1889, is visited by millions each year.

Her latest novel, which I finished last night, is absolutely gripping.

The meeting was postponed until Thursday, which suited everyone.

Our office, which overlooks the river, has fantastic views.

Notice that each sentence still makes sense if you remove the which-clause. The noun it refers to is already uniquely identified (The Eiffel Tower, her latest novel, our office).

Which to Refer to a Whole Clause

Which can also refer back to an entire preceding clause rather than a single noun — a construction not possible with that:

  • She passed all her exams, which surprised everyone. (which = the fact that she passed all her exams)
  • He arrived two hours late, which was frustrating.

Memory Trick

The most reliable memory trick uses commas as the deciding factor: if you need commas around the clause, use which; if you don’t need commas, use that. Think of it this way: which wraps itself in commas like a parenthetical remark, while that glues tightly to the noun with no punctuation gap.

A second trick: try removing the clause. If the sentence still means the same thing (the noun is still clearly identified), use which with commas. If removing the clause makes the sentence unclear or too vague, use that without commas.

A third approach: test with “by the way”. If the clause feels like a “by the way” addition, use which. If it is essential defining information, use that.

Common Mistakes

Mistake 1 — Using which without commas in a restrictive clause

✗ The report which I submitted yesterday contained an error.
✓ The report that I submitted yesterday contained an error.
(The clause is restrictive — it identifies which report; use that without commas.)

Mistake 2 — Using that in a non-restrictive clause

✗ London, that is the capital of England, has over nine million residents.
✓ London, which is the capital of England, has over nine million residents.
(The noun is already uniquely identified; the clause is non-restrictive — use which with commas.)

Mistake 3 — Forgetting the commas when using which non-restrictively

✗ My laptop which I bought in 2023 has a great display.
✓ My laptop, which I bought in 2023, has a great display.
(Non-restrictive which-clauses require commas before and after.)

Practice Links

Test your understanding of which vs that with interactive exercises:

More Confusing Words

Frequently Asked Questions

What is the difference between which and that in relative clauses?
That introduces a restrictive (defining) relative clause — essential to the meaning, no commas used. Which introduces a non-restrictive (non-defining) relative clause — adds extra information about something already identified, commas required. Example: "The car that I drive is old" (restrictive — tells you which car) vs "My car, which I bought in 2010, is old" (non-restrictive — adds a detail about a car already identified as "my car").
How do I know when to use that or which?
Use the removal test: try deleting the clause from the sentence. If the meaning changes significantly or the sentence becomes unclear, the clause is restrictive — use that (no commas). If the sentence still makes complete sense and the noun is still clearly identified, the clause is non-restrictive — use which (with commas). Another approach: if you would naturally add "by the way" before the clause, it is non-restrictive and takes which.
Can which be used without commas?
In standard British and American English, which in a non-restrictive clause always requires commas. American style guides (Chicago Manual of Style, AP Stylebook) are stricter and prefer that for restrictive clauses. British English is more flexible and uses which in both types of clause, with commas distinguishing non-restrictive use. For formal international writing or academic English, following the standard rule — that for restrictive (no commas), which for non-restrictive (with commas) — is safest and most widely understood.
Is "the house which Jack built" correct?
Grammatically, yes — in older and British English, which is widely used in restrictive clauses. "The house that Jack built" is the most familiar phrasing, and that is preferred by most modern style guides for restrictive clauses. "The house which Jack built" is not wrong, but "the house that Jack built" is the standard modern recommendation. The key point is that neither version uses commas, since both refer to a restrictive clause.
Can that introduce a non-restrictive clause?
No — that cannot be used in a non-restrictive clause. Writing "London, that is the capital of England, ..." is a grammar error. Non-restrictive relative clauses always use which (for things) or who/whom (for people). That is restricted to defining/restrictive clauses. This is one of the clearest rules distinguishing the two pronouns.
Can which refer to an entire clause, not just a noun?
Yes — which can refer back to an entire preceding clause or idea, something not possible with that. Examples: "She passed all her exams, which surprised everyone" (which = the fact that she passed); "He quit his job, which was unexpected." In these uses, which is sometimes called a sentential relative pronoun. Always use a comma before which when it refers to the whole preceding clause.
Do American and British English differ on which vs that?
Yes, slightly. American style guides (Chicago Manual of Style, AP Stylebook) require that for restrictive clauses and which (with commas) for non-restrictive. British English commonly uses which in both types of clause, with commas distinguishing non-restrictive use. For formal international writing, following the American rule — that for restrictive — is safest and most widely understood by international audiences.
What about using who instead of that or which for people?
For people, who (or whom) is preferred over that in relative clauses, though that is acceptable in informal speech. "The teacher who inspired me most retired last year" is more natural than "the teacher that inspired me most." That can be used for people in restrictive clauses, but who/whom is the stylistically preferred choice. Which is never used for people — only for things and sometimes animals.
Can that be omitted from a relative clause?
Yes — when that is the object (not the subject) of a restrictive relative clause, it can be omitted in informal writing: "The book [that] I read was excellent." However, when that is the subject of the clause, omission is not standard: "The man that called you" cannot drop that without altering the sentence completely. Which in non-restrictive clauses cannot be omitted because the commas and the clause are still structurally needed.
What is a quick memory trick for which vs that?
Remember: "THAT restricts, WHICH adds." Or use commas as a signal — which comes with commas, that does not. A visual trick: the word "which" is longer and takes up more space, just as a non-restrictive clause adds extra content without changing the core meaning. The word "that" is short and tight, just as a restrictive clause clamps down closely on the noun. When in doubt, apply the removal test: remove the clause — if the meaning survives intact, use which with commas; if the meaning changes, use that without commas.