Which vs That: Essential vs Non-Essential Clauses

Reviewed by the Grammarlyzer Editorial Team · Our Review Standards · Published January 22, 2026 · Updated June 13, 2026

Master the Comma Rule for Relative Clauses

📌 Quick Answer
That introduces essential information (no commas). Which introduces non-essential information (with commas). "The car that I bought is red." vs "The car, which is red, runs well."

Memory Trick: THAT = essential, no commas. WHICH = extra info, with commas.

💡 The Comma Test

If you can remove the clause without losing core meaning → use which + commas. If the clause is necessary → use that.

Quick Comparison

Word Clause Type Commas? Example Memory Tip
That Essential (Restrictive) No "The book that I read was great." Essential = That
Which Non-essential (Non-restrictive) Yes "The book, which was old, fell apart." Which = Extra (commas)

Common Mistakes

❌ Incorrect:

"The car which I bought broke down." (essential info, no commas)

✓ Correct:

"The car that I bought broke down."

Essential clauses use "that" without commas.
❌ Incorrect:

"My laptop that is three years old needs replacing."

✓ Correct:

"My laptop, which is three years old, needs replacing."

If you only have one laptop, the age is extra info → use "which" with commas.

🎯 Test Your Knowledge

Choose the correct word.

1. "The files ___ you need are on the server."

2. "The meeting, ___ was scheduled for 3 PM, has been postponed."

See It Live: Check a Sentence With Our Engine

Below is the same Harper engine that powers the homepage editor, running right on this page—no upload, no server round-trip. The starter sentence (“The car which I bought broke down." (essential info, no commas)”) already contains a slip—edit it or paste your own to watch the engine react.

The correct version is: "The car that I bought broke down.".

Honest limits: Which and That are both correctly spelled words, so a checker often can't tell which one you meant (Essential vs Non-Essential Clauses). That decision is yours—use the rule above, then run the check for the errors it can catch.

Frequently Asked Questions

What is the difference between which and that?

"That" introduces essential clauses necessary for meaning. "Which" introduces non-essential clauses that add extra information and require commas.

When do I use commas with which or that?

Use commas with "which" (non-essential). Don't use commas with "that" (essential).

When to Use "That"

Professional Examples

  • "The report that you requested is attached." (essential info)
  • "Policies that affect employees require review."

Academic Examples

  • "Studies that use large samples are more reliable."
  • "The theory that explains this phenomenon is new."

Casual Examples

  • "The movie that we saw was amazing."
  • "I need the tools that are in the garage."

When to Use "Which"

Professional Examples

  • "The new policy, which was approved last week, takes effect Monday."
  • "Our flagship product, which launched in 2020, remains popular."

Academic Examples

  • "The study, which involved 500 participants, was conclusive."
  • "This methodology, which is widely accepted, ensures accuracy."

Casual Examples

  • "My phone, which I just bought, already cracked."
  • "The pizza, which was cold, still tasted good."

Word Origins & Etymology

Which comes from Old English 'hwilc' (which one, of what kind). It introduces non-restrictive (extra info) clauses set off by commas.

That derives from Old English 'þæt' (the neuter demonstrative). It introduces restrictive (essential) clauses with no commas.

🔗 The Connection

The which/that distinction is primarily an American English convention; British English often uses 'which' for both restrictive and non-restrictive clauses.

Real-World Examples

📏 Restrictive:

The book that I ordered arrived today.

That = essential info (which book? the one I ordered)
📏 Non-restrictive:

The book, which I ordered last week, arrived today.

Which + commas = extra info (we already know which book)
💼 Business:

The proposal that the board approved will launch in Q3.

That = identifies which proposal
💼 Business:

The proposal, which took three months to prepare, was unanimously approved.

Which = adds bonus detail
❌ Mistake:

The car which I bought last year needs new tires.

Debatable: American style prefers 'that' for restrictive clauses. British English accepts 'which' here.
💡 Test:

Can you remove the clause without losing the sentence's meaning? → Use which + commas. Does removing it change which item you mean? → Use that, no commas.

The removal test is the most reliable method

Why Do People Confuse Them?

This rule is a style convention, not a hard grammatical law. British English uses 'which' for both restrictive and non-restrictive clauses freely. American style guides enforce the distinction. The takeaway: in American formal writing, use 'that' for essential info (no commas) and 'which' for bonus info (with commas).

Practice with Related Guides

Keep practicing with closely related guides: Who vs Whom and Relative Clauses Guide.

Related Articles

Which and That in Real-World Documents

In business writing, the which/that distinction is one of the clearest markers distinguishing edited professional prose from casual writing. "That" introduces restrictive relative clauses — clauses that are essential to the identity of the noun they modify and cannot be removed without changing the sentence's meaning. "Which" introduces non-restrictive clauses — clauses that add supplementary information about an already-identified noun and are set off by commas. A contract clause like "The provisions that apply to non-residents are described in Appendix B" restricts the scope to a specific subset of provisions. Changing it to "The provisions, which apply to non-residents, are described in Appendix B" suggests all provisions apply to non-residents — a potentially significant legal difference.

In academic writing, where precision of reference is critical to the validity of arguments and the accurate description of research, the which/that distinction helps writers control exactly how much information they claim about their subjects. "The studies that controlled for confounds showed significant effects" restricts the claim to one subset of studies. "The studies, which controlled for confounds, showed significant effects" implies all studies in the discussion controlled for confounds. These are substantively different claims, and the wrong choice can misrepresent research findings. American academic style — guided by the Chicago Manual of Style, APA, and MLA — consistently enforces the restrictive/non-restrictive distinction using "that" and "which" respectively.

British English historically tolerated "which" in both restrictive and non-restrictive clauses, which is why some writers trained in British schools or reading British texts find the "that for restrictive" rule unfamiliar. Contemporary British style guides have increasingly adopted the American distinction, but inconsistency persists in British academic and journalistic writing. For American publication, the rule is strict: restrictive clauses use "that" (no comma), and non-restrictive clauses use "which" (preceded by a comma). Writers editing their own work should search for every "which" and ask whether the clause it introduces could be removed without affecting the sentence's core meaning — if not, convert to "that."

The Comma Test for Which and That

If a relative clause is set off by commas, it is non-restrictive and should use "which." If there are no commas and the clause is essential to identifying the noun, it is restrictive and should use "that." Read the sentence aloud after removing the clause: if the noun is still clearly identified, the clause is non-restrictive (which). If the meaning changes or becomes vague, the clause is restrictive (that).

Questions About Choosing Which vs. That

What is the practical difference between a restrictive and non-restrictive clause?

A restrictive clause limits or defines which specific noun the speaker is referring to — it restricts the reference to a subset. "The car that she bought last year is already broken" uses "that" because the clause identifies which car: not any car she has ever owned, but the one she bought last year. Remove the clause and the sentence becomes vague. A non-restrictive clause adds information about a noun already uniquely identified: "Her new car, which she bought last year, is already broken" adds the purchase date as background detail. The noun "her new car" is already specific without the clause. This is the test: can the noun be uniquely identified without the clause? If yes, use "which" with commas. If no, use "that" without commas.

Can "which" ever be used without commas?

In formal American English, "which" in a relative clause should always be preceded by a comma (or a dash) because it signals a non-restrictive clause. Without commas, the sentence implies a restrictive meaning that should use "that." However, "which" appears without commas in several other grammatical constructions that are not relative clauses: interrogative uses ("Which route is faster?"), indirect questions ("Tell me which option you prefer"), and free relative clauses ("Take which you like"). In these constructions, "which" is not introducing a relative clause modifying a specific noun, so the comma rule does not apply. Only when "which" introduces a relative clause following a specific noun does the comma-and-non-restrictive rule apply.

Does British English have the same which/that rule?

Not strictly. British English traditionally used "which" for both restrictive and non-restrictive clauses, with context and commas distinguishing them. Modern British style guides — including the Oxford Style Guide — now recommend using "that" for restrictive clauses in line with American practice, but the change is not uniformly applied, and British academic and literary writing still frequently uses "which" restrictively. Writers preparing manuscripts for American publishers, journals using APA or Chicago style, or any American audience should follow the American rule consistently. Writers targeting British publications should consult the target publisher's specific style guide, as preferences vary by publication.

How do I fix which/that errors efficiently during editing?

Use the Find function to locate every "which" in the document. For each instance, check whether it introduces a relative clause modifying a specific noun. If it does, ask: is this clause essential to identifying the noun, or is it supplementary information? If essential (restrictive), change "which" to "that" and remove the preceding comma. If supplementary (non-restrictive), keep "which" but ensure there is a comma before it. Then perform a second pass searching for "that" and verify each restrictive usage — some "that" clauses may actually be non-restrictive if the noun is already uniquely identified by other context, in which case they need commas and conversion to "which."

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