Which and that are relative pronouns used to add extra information to a sentence. That is used for essential (restrictive clauses), while which is used for non-essential (nonrestrictive clauses). People confuse them because both can refer to things and often sound natural in speech. Different grammar styles, like those from the Chicago Manual of Style and Oxford English Dictionary, also create mixed usage.
After reading this guide, you will understand the correct rule, how to avoid mistakes, and when each word fits best in formal and everyday English.
Quick Answer
Use that for important information. Use which for extra information.
Examples:
- The book that I bought is expensive.
- The book, which I bought yesterday, is expensive.
In the first sentence, the clause is essential. In the second, it is extra.
Think of it this way:
- That = needed information
- Which = added detail (with commas)
This rule is common in grammar rules, English writing, and formal writing.
The Origin of Which or That
The words which and that come from Old English. That comes from “þæt,” meaning a general reference word. It has always been used for defining or limiting meaning. Which comes from “hwilc,” meaning “what kind.” Over time, which became linked to adding descriptive detail.
Modern grammar guides like the Chicago Manual of Style prefer a clear difference:
- That for restrictive clauses
- Which for nonrestrictive clauses
However, British usage sometimes allows overlap. This history explains why confusion still exists in English grammar today.
British English vs American English Spelling
Both British and American English use which and that, but rules are stricter in American English.
| Usage Type | American English | British English |
|---|---|---|
| Restrictive clause | That preferred | That or which |
| Nonrestrictive clause | Which with commas | Which with commas |
| Formal writing | Strict rule followed | More flexible |
Examples:
- US: The car that I bought is new.
- UK: The car which I bought is new.
In formal writing, especially academic or business writing, the American rule is often followed globally.
Which Spelling Should You Use?
Choose based on your audience:
- US audience → Use strict rule (that = essential, which = extra)
- UK audience → Flexible, but still safer to follow rule
- Global audience → Use clear rule for better understanding
If you want safe and professional writing, follow this rule:
Use that for defining clauses and which with commas for extra detail.
This approach improves clarity in content writing, SEO writing, and academic writing.
Common Mistakes with Which or That
Many writers mix these words incorrectly. Here are common mistakes:
Wrong: The house which I built is big.
Correct: The house that I built is big.
Wrong: The car that, I bought yesterday, is fast.
Correct: The car, which I bought yesterday, is fast.
Key mistakes:
- Using which without commas for essential info
- Using that with commas
- Ignoring clause type
Tip: If you can remove the clause without changing meaning, use which.
Which or That in Everyday Examples
Email:
The file that you sent is missing data.
Social media:
This is the phone that everyone wants.
News:
The report, which was released today, shows growth.
Formal writing:
The method that we used improved results.
These examples show how both words appear in real-world writing, including business communication, digital content, and academic text.
Which or That – Google Trends & Usage Data
Search trends show that “which vs that” is a common query worldwide. Countries like the US, UK, India, and Pakistan search this topic often due to confusion in English grammar learning.
In Google Trends, the keyword appears frequently in:
- Students learning grammar
- Content writers
- Bloggers and SEO experts
This proves that understanding this rule improves writing clarity and professional communication.
Comparison Table: Which vs That
| Feature | That | Which |
|---|---|---|
| Clause type | Restrictive | Nonrestrictive |
| Importance | Essential | Extra |
| Commas | No | Yes |
| Usage | Defines meaning | Adds detail |
| Example | The book that I read | The book, which I read |
FAQs
1. Can I use which instead of that?
Yes, but only in non-essential clauses with commas.
2. Is “that” always correct?
It works in most cases, especially essential clauses.
3. Why do British writers use “which” more?
British English allows more flexibility in grammar usage.
4. How do I test which one to use?
Remove the clause. If meaning changes, use that.
5. Is this rule important for SEO writing?
Yes, clear grammar improves readability and ranking.
6. Do style guides agree on this rule?
Most, like the Chicago Manual of Style, support it.
7. Is spoken English different?
Yes, people often mix both in casual speech.
Conclusion
That is used for essential information, while which adds extra detail with commas. This simple rule improves clarity in writing. Always check if the clause is necessary before choosing. A practical tip is to remove the clause and see if the sentence still makes sense.
If it does, use which. For better grammar skills, you can also read our guide on affect vs effect to strengthen your understanding of confusing English words.
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