Mastering the choice between which and that in sentence construction

One of the most common issues I correct when proofreading academic and professional writing is the misuse of which and that. While the rule itself is straightforward, getting it wrong can affect clarity, tone, and—in academic work—perceived credibility.

If you’ve ever paused mid-sentence and wondered whether to write which or that—you’re not alone. These tiny words can cause big confusion. The difference often hinges on a single, subtle detail: the comma. So when should you use 'which', when do you leave the comma out, and when is 'that' the better choice?

Does your sentence need 'which' or 'that'?

Let’s break it down.

Use 'that' for essential information (no comma)

When a clause is essential to the meaning of a sentence—meaning the sentence wouldn’t make complete sense without it—use 'that'. These are known as restrictive clauses.

Examples:

• The book that I borrowed from the library was fascinating.

• She wore the dress that she bought last summer.

In both cases, the part after “that” is crucial. It tells us which book or which dress. If we removed the clause, the sentence would lose meaning or become vague.

Use 'which' with a comma for extra, non-essential detail

When the clause adds extra information that isn’t essential to the meaning, use 'which' with a comma. These are called non-restrictive clauses.

Example:

• Her car, which she bought just last year, has already broken down twice.

In this case, which she bought just last year is extra information. The main point is that the car has broken down; the fact that she bought it last year is interesting but not essential. If we removed the clause, the sentence would still be clear:

Her car has already broken down twice.

In this example, the bit after 'which' adds a detail, but it’s not necessary for identifying the subject. If you removed the clause, the sentence still makes sense.

Tip: If you can take the clause out without changing the meaning, use 'which' with a comma.

Can you use 'which' without a comma?

Technically, yes—but only in British English, and even then it’s often considered informal or stylistically inconsistent.

In British usage, you’ll sometimes see restrictive clauses with 'which' and no comma:

• The house which Jack built is still standing.

However, for clarity and consistency—especially in formal writing—it’s still best to reserve 'which' for non-restrictive clauses with a comma, and 'that' for restrictive clauses.

Pro tip: If you're writing for an international or mixed audience, sticking to the 'that = restrictive / which = non-restrictive with comma' rule is the safest bet.

Why This Matters in Academic and Professional Writing

In essays, dissertations and published work, small grammatical errors can distract the reader and weaken an otherwise strong argument. Unlike spellcheckers, automated tools often fail to identify whether a clause is restrictive or non-restrictive—which is where human proofreading becomes essential.

A quick recap

Use This	When	Example
that	The clause is essential (restrictive)	The cake that she baked was delicious.
which,	The clause adds extra, non-essential detail (non-restrictive)	The cake, which she baked yesterday, was delicious.
which (no comma)	Informal or British English usage (less common in formal writing)	The car which broke down was sold.

Want your writing to be crystal clear?

If you're unsure whether you're using 'which' or 'that' correctly—or want the reassurance of an expert eye—KT Proofreading can help. I offer a professional proofreading and editing service, specialising in British English, supporitng authors, academics and professionals in refining writing.

Whether you're polishing an academic essay, preparing professional content, or fine-tuning fiction before submission or publication, I can spot the small details that make a big difference.

Get in touch today for professional proofreading and editing support—because good grammar should never get in the way of a great idea.

About the author

Kat Taylor is a professional proofreader and editor based in the UK, specialising in British English. She works with authors, academics and professionals to help refine their writing for clarity, accuracy and confidence. Learn more about Kat or explore her proofreading and editing services.

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