In certain sentences, I use that is to rephrase something, be more specific or elaborate.


This coffee is so black, it must have been prepared by someone who works a lot, that is: an insomniac.

But in certain circumstances, this colon is unpractical because there's another colon around. Or it makes the sentence insist too much on this particular point.

And I think that I already saw that is without a colon.

Is this punctuation sign required?

  • 1
    I guess one could write: ‘[…] someone who works a lot, that is, an insomniac.’ However, writing it without a colon or a comma seems strange and wrong — at least to me. Also, I think a comma is more natural than a colon.
    – rberaldo
    Commented May 10, 2011 at 22:03
  • The comma is nice, thanks! It's lighter and may well resolve the emphasis problem that I mention.
    – olivierg
    Commented May 10, 2011 at 22:08

3 Answers 3


Anecdotally, I have only rarely seen people put a colon in that position. The colon implies a certain logical relationship between the clauses, namely that the second clause is an illustration or defining example of the first clause.

A comma is more common. It is a textual analog of a brief pause in speech. Since a pause in speech can be used for any one of many purposes -- for opening a new thought, for indicating the speaker's uncertainty, for dramatic effect -- it cannot carry quite the same weight of logic as the more specialized colon.

I can see either being used with "that is", but the comma seems more generic, and thus safer.

  • I think that the comma is what I was searching for, thank you. But you don't mention whether punctuation is required at all. I am pretty sure that I saw it without any punctuation. Is this necessarily a mistake? (my "insomniac" example is maybe not adapted to illustrate this, because it talks about a person, and it doesn't play well with "that").
    – olivierg
    Commented May 10, 2011 at 22:16
  • 1
    There are grammatical constructions using "that is" which do not have any punctuation after it. For example, Shakespeare often wrote things like The cock, that is the trumpet to the morn, (Hamlet, I.i.173) which lack any punctuation after the "is". So you could say something like "The cat, that is Felix, caught a mouse". Saying "The cat that is Felix caught a mouse" without any punctuation anywhere would sound odd though. More declarative than exemplary. But I don't think there's a hard or fast rule on this one. Commented May 10, 2011 at 22:28
  • Thank you, my question seems fully answered now. I know the possibilities and will use them according to the context and my inspiration :) I will wait a bit before accepting your answer, to see if others have something else to say about this.
    – olivierg
    Commented May 10, 2011 at 23:12

How about trying

This coffee is so black, it must have been prepared by someone who works a lot; that is, an insomniac.

It clarifies your point without insisting too much.

  • I think that I understand you point. By splitting the sentence with a semicolon, the ending doesn't look too much like a conclusion, which was indeed the problem. That said, my example isn't perfect. One big problem that I had, and that I didn't illustrate, is when the "that is" construct appears in the middle of the sentence. In such cases, I don't want to break the rhythm, "that is" is only meant to precise a detail. This is also what I tried to say about insisting too much. In these cases a colon would split the sentence at the wrong place.
    – olivierg
    Commented May 12, 2011 at 13:08

Use of the colon implies the introductory phrase "that is,"which clarifies or emphasizes the independent/complete clause that precedes the colon. The phrase that follows the colon (which can be a dependent or independent clause)is often in the form of a list or appositive. For the given example,try this: This coffee is so black, it must have been prepared by someone who works a lot: an insomniac. This way (in which the dependent clause following the colon is an appositive) there is the implication of the phrase "that is," in the connection between "someone who works a lot" and "an insomniac."

Here is another example:

Punctuation is fascinating only to those who have no other lives: English teachers.

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