That vs. Which – Defining vs. Non-Defining Clauses

Understanding the difference between “that” and “which” can be one of those grammar points that confuses even the most fluent English speakers. But here’s the good news.

Once you understand what defining and non-defining clauses really are and how these two words behave in sentences, the confusion clears up quickly. The key lies in knowing what information is necessary to the meaning of the sentence and what is just extra detail.

Let’s break it down in a way that works for everyone, whether you’re a student writing an essay, a professional sending emails, or just someone who loves clean and clear communication.

What Are Clauses?

A clause is simply a group of words that contains a subject and a verb. In the context of “that” vs. “which,” we are mostly talking about relative clauses. These clauses give more information about a noun and are introduced by words like “that,” “which,” “who,” or “whom.”

Now, there are two main types of relative clauses we need to focus on here:

1. Defining clauses (also called restrictive clauses)

2. Non-defining clauses (also called non-restrictive clauses)

Understanding these two types is the first big step toward using “that” and “which” correctly.

READ: Avoiding Run-On Sentences with Simple Fixes

Defining Clauses – Use “That”

A defining clause gives essential information. Without this clause, the meaning of the sentence would be incomplete or unclear. You use “that” in defining clauses. You do not place a comma before “that.”

Example 1:

The book that has a red cover is mine.

In this sentence, the clause “that has a red cover” tells us which book we’re talking about. If we remove it, we just get “The book is mine,” which could refer to any book. The clause is necessary.

Example 2:

The woman that lives next door is a nurse.

Here, we need to know which woman we’re talking about. Without the clause, the sentence becomes too vague.

Example 3:

Please pick the shirt that fits best.

There may be several shirts. The clause helps identify the correct one.

Example 4:

He’s the player that scored the winning goal.

The clause helps us know which player we’re referring to. It’s a specific person.

So, anytime the information is essential to identifying the noun, use “that” without a comma.

Non-Defining Clauses – Use “Which”

A non-defining clause gives extra information. This information is helpful but not essential to identify the noun. The sentence would still make perfect sense without it. For these clauses, use “which” and place a comma before and after the clause.

Example 1:

My car, which I bought last year, has a new dent.

You could remove the clause “which I bought last year”, and the sentence would still make sense: “My car has a new dent.” The extra information adds context, but it’s not necessary.

Example 2:

We visited the museum, which was recently renovated.

You already know which museum. The clause gives extra detail.

READ: Avoiding Comma Mistakes Before “Because”

Example 3:

The letter, which arrived late, caused a lot of confusion.

Removing the clause still leaves you with a full sentence.

Example 4:

Her laptop, which she uses for graphic design, crashed suddenly.

Again, you already know what “her laptop” is. The clause is just giving you more insight.

Use “which” when you’re adding extra information that the sentence can live without.

Visual Comparison Chart

Here’s a simple guide to help you remember when to use “that” vs. “which.”

Why This Matters in Everyday English

Using “that” and “which” correctly helps make your writing clearer and more professional. It avoids misunderstandings. Especially in formal writing, such as business emails, research reports, and legal documents, using the right word can make your message more precise.

Let’s consider this example:

The software that crashed caused the delay.
This means the specific software that crashed caused the delay — not other software.

Now try:
The software, which crashed, caused the delay.
This means the software crashed, but you’re just giving more info. Any software would have caused the delay.

See how one word can change the meaning?

Common Mistakes People Make

1. Using “which” when “that” is needed:
   Incorrect: The car which is parked outside is mine.
   Better: The car that is parked outside is mine.

2. Using commas with “that”:
   Incorrect: The phone, that I bought, is expensive.
   Correct: The phone that I bought is expensive.

3. Using “that” in non-defining clauses:
   Incorrect: My laptop, that broke down, is still under warranty.
   Correct: My laptop, which broke down, is still under warranty.

4. Assuming “which” is always more formal:
   Some people think using “which” sounds smarter, but formality isn’t the issue. Clarity is.

READ: Correct Use of “Who” vs. “Whom” – A Simple, Practical Guide for Everyone

Quick Rule Recap

• If the clause is essential to identify something, use “that” with no comma.

• If the clause is just extra detail, use “which” with a comma.

• Always check if removing the clause changes the meaning of the sentence. If it does, it’s defining, and you should use “that”.

Frequently Asked Questions (FAQ)

1. Can I use “which” instead of “that” in American English?

American English is stricter about the distinction. It prefers “that” for defining clauses. British English is more flexible and sometimes uses “which” even in defining clauses, but the safest rule — especially in formal writing — is to stick with “that” for defining clauses and “which” for non-defining ones.

2. What if I just use “which” all the time?

You risk sounding unclear. “Which” with no comma can confuse readers. It also makes your writing less precise. If you’re taking an exam or writing professionally, mixing them up can cost you points or credibility.

3. Can I leave out “that” in a sentence?

Yes, sometimes in casual speech or informal writing, “that” can be omitted in defining clauses.
Example: The cake she made was delicious.
But in formal writing, it’s better to include it: The cake that she made was delicious.

READ: Affect vs. Effect Clarified with Real Examples

4. Is there ever a time when both “that” and “which” are correct?

Yes, when writing informally or when the sentence structure is simple and both words could technically work. But clarity and punctuation will decide which one is better. Stick with the defining/non-defining rule to stay safe.

5. Do “that” and “which” ever refer to people?

No. When referring to people, you use “who” for both defining and non-defining clauses.
Example: The teacher who taught me English was kind.
Example: My brother, who lives in Canada, is visiting soon.

Final Thoughts

Getting “that” and “which” right isn’t just about following a grammar rule. It’s about making your writing clear, direct, and easy to understand. When you understand defining and non-defining clauses, you don’t just memorize grammar—you master communication. You start to write in a way that respects your reader’s time and attention.

Think of it like giving directions. If you say, “Take the bus that stops at the library,” you’re pointing to a very specific bus. But if you say, “The bus, which stops at the library, is often late,” you’re just throwing in a little side note. One helps someone catch the right bus. The other just helps them prepare to wait.

Choose wisely. Your reader will thank you.