That vs Which? [Difference explained]

Mary Cullen
Post by Mary Cullen
Originally published January 24, 2021, updated December 22, 2023
That vs Which? [Difference explained]

One of the trickier grammar rules in the English language is the difference between the words that and which. Both serve a similar purpose of clarifying the information in a sentence. As you develop your business writing skills, using each word correctly will strengthen your communication. Today, we'll discuss when to use that and which.

Let’s start with the similarities between these words (and some background information).

That and which both introduce adjective clauses. A clause is any group of words that includes a noun and a verb. There are independent clauses (full sentences such as “I like to eat ice cream”) and dependent clauses (incomplete sentences that do not express a complete idea, such as “When I get to school …”)

Adjective clauses are dependent clauses that describe nouns, i.e. they act like adjectives. However, in English, adjectives come before the noun and adjective clauses come after the noun. Adjective clauses often start with the words who, whom, whose, where, when, which, and that. Here is an example of each (the adjective clauses are underlined):

ACME Biochemical is one of the places where I want to work.  

Where I want to work” describes places. 

Tom Jones is the man whose dog attacked me.

Whose dog attacked me” describes the man. 

John and Sarah are the people whom I would like to work with on this project. 

Whom I would like to work with” describes the people.

Divisions that generate profit often glean admiration.

That generate profit” describes divisions. 

Shinzo Abe, who was Japan’s longest-serving prime minister, was assassinated in July 2022. 

Who was Japan’s longest-serving prime minister” describes Shinzo Abe. 

We should all pay attention to David’s new product idea, which is likely to triple sales next year. 

Which is likely to triple sales next year” describes David’s new product idea.

Look at the examples again. Do you notice something different about the last two examples? The punctuation is different; the adjective clause is contained within commas. This gives you a hint at the difference between which and that, and how to use that or which correctly.  


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The difference between which and that

Their difference is that one is used to present essential information in the sentence and the other is used to present non-essential information in the sentence. 

  • That introduces essential information in a “restrictive clause.” This is also called an essential clause.
    Example: Divisions that generate profit often glean admiration.
  • Which introduces extra information in a “nonrestrictive clause.” This is also called a nonessential adjective clause.
    Example: We should all pay attention to David’s new product idea, which is likely to triple sales next year.

Let's dive deeper into this challenging grammar concept, and how you can immediately determine the correct answer for that or which every time you construct a sentence.


Below is the correct usage of that:

Divisions that generate profit often glean admiration.

The words that generate profit restrict the divisions you're talking about. Without this restrictive clause of the specific divisions, the meaning of the sentence would change. Without clarification, you'd be stating that all divisions glean admiration, not just the profitable divisions.

The clause “that generate profit” is essential to the meaning of the sentence, so the correct word is that. You cannot remove that clause without changing the meaning of the sentence. Also, a comma is never needed with that in a restrictive clause.

Which example

Here's the correct usage of which:

We should all pay attention to David’s new product idea, which is likely to triple sales next year.

The clause following which is likely to triple sales next year adds extra information to the sentence. They are nonrestrictive, which means that you can remove them and the sentence will still have the same meaning. “David’s new product idea” is clear and specific enough without the which clause, so it's a nonrestrictive clause/nonessential clause. And when you don’t need the information in the adjective clause, you do need the commas. 


That vs. Which
punctuation - an inverse relationship

A comma is never needed with that in a restrictive clause. But in the case of which, you always need the commas. This is confusing, but I always remember it as an inverse relationship:

If you NEED the information, you DON’T NEED the commas. That introduces necessary information. You need the information for the sentence to make sense, so you do not need the commas.

If you DON’T NEED the information, you NEED the commas. Commas set off extra, unnecessary information. Which is always used with commas in an adjective clause.

**These punctuation rules apply to the other adjective clauses as well - those that start with who, whom, when, where, and whose. If you need the info, don’t use commas. If it’s extra info, make sure to use those commas!

Recap: Key takeaways about using that vs which

Quick review time! Remember these key ideas to help you choose that or which and use them correctly:

  1. That and which both introduce adjective clauses. 
  2. Adjective clauses come after the noun they are describing. 
  3. That is used with information you NEED (called a restrictive clause or essential adjective clause). Do not use commas with that. Which is used with EXTRA information (called a non-restrictive clause or non-essential adjective clause). Always use commas with which. 
  4. The punctuation rules for which and that apply to other types of adjective clauses as well. 

If this or other grammatical quirks are preventing you from making progress in your business writing, a business writing course with individualized feedback is a great place to start your path to stronger, clearer business writing!

Enroll in our online, self-paced Business Writing Techniques for Non-Native English Writers Course for writing tips and guidance. Our Proofreading and Grammar course will also help ensure your finished documents are error-free every time. 


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Mary Cullen
Post by Mary Cullen
Originally published January 24, 2021, updated December 22, 2023
Mary founded Instructional Solutions in 1998, and is an internationally recognized business writing trainer and executive writing coach with two decades of experience helping thousands of individuals and businesses master the strategic skill of business writing. She excels at designing customized business writing training programs to maximize productivity, advance business objectives, and convey complex information. She holds a B.A. in English from the University of Rhode Island, an M.A. in English Literature from Boston College, and a C.A.G.S. in Composition and Rhetoric from the University of New Hampshire.