Rule 1: You should put a comma before and after “that is” when you are introducing a dependent list of things or information to offer clarification.
- Correct: I am great at cooking Chinese food, that is, sweet and sour chicken and fried rice.
- Incorrect: I am great at cooking Chinese food that is sweet and sour chicken and fried rice.
Rule 2: You should put a comma after “that is” and a semicolon before when it appears before an independent clause.
- Correct: She is a trained sushi chef; that is, she studied at a culinary school in Japan for ten years.
- Incorrect: She is a trained sushi chef, that is she studied at a culinary school in Japan for ten years.
Rule 3: You should not use a comma with “that is” when you use it to specify “what something is” or “why something is.”
In these cases, the term “that is” is essential for the sentence to make sense.
- Correct: I don’t have any money, and that is why I didn’t come.
- Incorrect: I don’t have any money, and, that is, why I didn’t come.
Please keep reading the rest of the page so you can learn more about using commas with the term “that is.”
When to Use a Comma With “That Is”
If you are deliberating over when to use a comma with “that is” you should refer to Rules 1 and 2.
Rule 1: Use a comma before and after “that is” when it introduces extra information that is not essential for the sentence to make sense.
However, to have commas around “that is” requires that the information after it is not independent. In this context, the term “that is” means the same as “specifically” or “namely.”
- He has traveled throughout the Americas, that is, Brazil, Argentina, Peru, and Chile.
Furthermore, you can add the words “to say” after “that is,” and the meaning is the same.
- The director of the company, that is to say, my boss, is placing more and more demands on staff.
Rule 2: Use a comma after “that is” when it precedes an independent clause that can stand alone without requiring other words.
On these occasions, you should also use a semicolon before “that is.”
- He is highly competent at stock trading; that is, he makes a lot of money buying and selling shares.
- I love animals; that is to say, I have three dogs and two cats.
When to Avoid a Comma With “That Is”
If you want to know when you should avoid using commas with “that is,” please check Rule 3.
Rule 3: You shouldn’t use a comma after “that is” when the sentence needs it to make sense
In these sentences, the term “that is” generally refers to “what” or “why” something is the way it is.
- I have doubts about whether that is our best option.
- Paul is sick. So, I wonder if that is why he didn’t come last night.
Conclusion
Use a comma before and after “that is” when it introduces a dependent element. Use a semicolon before and a comma after when the clause after “that is” is independent. Do not put a comma when “that is” is essential for the sentence to make sense.