Selecting the right relative pronoun often feels like a guessing game. Who, that, and which are the three most common words used to add information to a sentence, but swapping them incorrectly can change your meaning or make your writing look unprofessional. The difference primarily comes down to whether you are talking about a human or a thing, and whether the information is essential or just extra detail.
You don’t need a degree in linguistics to master this. By following a few standard conventions, you can ensure your sentences are always grammatically sound and easy to read.
The Golden Rule: Humans vs. Things
Before worrying about commas or complex grammar clauses, start with the simplest distinction: Is the subject alive and human?
Who
Exclusively for People
Use who when you are referring to a person or a group of people.
Example: “The woman who called me was very helpful.”
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That / Which
For Objects & Animals
Use that or which when referring to inanimate objects, concepts, or animals.
Example: “The phone that rang is mine.”
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While this covers the basics, English has a few specific nuances. Let’s break them down further.
When to Use “Who”
Who is the correct choice for people. It keeps the sentence personal and grammatically accurate. While you might hear people say “The girl that won,” standard grammar rules prefer “The girl who won.”
- Correct: “I know the man who wrote this book.”
- Incorrect: “I know the man which wrote this book.”
Pro Tip for Pets: If you are writing about a pet with a name and a personality, it is acceptable to use who. For example: “My dog Rover, who loves bacon, is sleeping.” If it is a generic animal, stick to that or which.
The Battle: “That” vs. “Which”
This is where most writers get stuck. Both words refer to things, but they are not interchangeable in formal writing. The choice depends on commas and the importance of the information.
1. “That” is for Essential Information (Restrictive)
Use that when the phrase is necessary to understand the sentence. If you remove the phrase, the sentence loses its core meaning or specificity. These clauses are never separated by commas.
- “The bike that has a flat tire needs to be fixed.”
In this example, we aren’t talking about any bike. We are talking specifically about the one with the flat tire. The phrase restricts the meaning.
2. “Which” is for Extra Information (Non-Restrictive)
Use which when you are adding interesting but non-essential details. If you remove the phrase, the sentence still makes perfect sense. These clauses must be surrounded by commas.
- “My bike, which has a flat tire, is in the garage.”
Here, the core sentence is “My bike is in the garage.” The fact that it has a flat tire is just extra detail.
The “Comma Test”
If you can put commas around the phrase, use which. If commas look wrong, use that.
Think of it like a disposable wrapper: If you can throw the phrase away without breaking the sentence, use commas and which.
Comparison Table
Here is a quick reference guide to help you choose the right word every time.
| Word | Refers To | Commas? | Clause Type |
|---|---|---|---|
| Who | People | Depends | Essential or Extra |
| That | Things / Animals | NO | Essential (Restrictive) |
| Which | Things / Animals | YES | Extra (Non-restrictive) |
Common Confusions and Exceptions
Even native speakers slip up on these rules. Watch out for these specific scenarios.
Collectives (Teams and Companies)
Groups of people, like a “team,” “band,” or “company,” are grammatically treated as singular things in American English. Therefore, you should usually use that or which.
- “The company that sells these shoes is famous.” (Preferred)
- “The company who sells these shoes…” (Often considered incorrect)
“That” referring to People
In casual conversation, you will often hear: “He is the guy that helped me.” While this is becoming accepted in modern spoken English, editorial standards and formal writing still demand who.
Formal Writing Checklist
- ✅ Person → Who
- ✅ Thing + No Comma → That
- ✅ Thing + Comma → Which
