Which and that look similar, function similarly, and are so often used interchangeably that many writers do not realise there is a meaningful difference between them. In careful British English writing, however, the distinction matters — particularly in formal, academic and professional contexts.
This guide explains exactly when to use which and when to use that, covers the role of commas in making the distinction, and gives you clear examples to follow.
Use that for restrictive clauses — clauses that are essential to the meaning of the sentence.
Use which for non-restrictive clauses — clauses that add extra information but are not essential to the meaning.
A practical shortcut: if you can remove the clause without changing the core meaning of the sentence, use which and put commas around it. If removing the clause would change the meaning, use that with no commas.
A restrictive clause defines or limits the noun it refers to. Without the clause, the sentence would mean something different or would be incomplete. Use that for restrictive clauses — no commas required.
The report that was submitted on Monday has been approved.
Here, “that was submitted on Monday” is essential to the sentence. It identifies which report was approved. Without it — “The report has been approved” — we do not know which report. The clause is restrictive, so we use that.
The documents that contain errors need to be corrected before submission.
Again, the clause “that contain errors” is essential — it identifies which documents. Remove it and the meaning changes entirely.
A non-restrictive clause adds extra information about a noun, but removing it would not change the core meaning of the sentence. Use which for non-restrictive clauses — always with commas.
The report, which was submitted on Monday, has been approved.
Here, “which was submitted on Monday” is additional information — interesting, but not essential to identifying which report. The sentence “The report has been approved” still makes complete sense without it. The clause is non-restrictive, so we use which, with commas.
The final chapter, which covers the conclusions, is the most important section.
The clause “which covers the conclusions” adds information but is not essential to identifying which chapter. Use which, with commas.
The comma is the clearest signal of the which/that distinction:
If you are unsure which word to use, ask yourself: can I put a comma before this clause? If yes, use which. If no, use that.
It is worth noting that in American English, the which/that distinction is applied more strictly than in British English. American style guides — particularly the Chicago Manual of Style — are very firm that which should only be used for non-restrictive clauses. British style guides are somewhat more flexible, and in informal British writing, which is often used for both restrictive and non-restrictive clauses.
However, in formal British academic and professional writing, maintaining the distinction is considered good practice and marks out careful, precise writing. For any formal document, following the rule — that for restrictive, which for non-restrictive — is the safest and most credible approach.
In informal British English, which is sometimes used for restrictive clauses without any confusion. However, in formal academic and professional writing, maintaining the distinction — that for restrictive, which for non-restrictive — is considered best practice and reflects careful, precise writing.
Ask yourself: if I remove this clause, does the sentence still make sense and mean the same thing? If removing it changes the meaning or makes the sentence incomplete, the clause is restrictive — use that. If removing it leaves the sentence perfectly intact and meaningful, the clause is non-restrictive — use which, with commas.
In formal writing, a which without a comma before it is usually a sign that that would be more appropriate. The comma is what signals to the reader that the clause is non-restrictive. Without the comma, which can create ambiguity about whether the clause is essential or supplementary.
No. For people, use who for both restrictive and non-restrictive clauses — not that or which. The student who submitted early received her feedback first. (restrictive) Professor Smith, who has published extensively on this topic, supervised the project. (non-restrictive)
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