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

Example:

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?

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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 May 10 '11 at 22:03
    
The comma is nice, thanks! It's lighter and may well resolve the emphasis problem that I mention. –  olivierg May 10 '11 at 22:08
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2 Answers

up vote 6 down vote accepted

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.

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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 May 10 '11 at 22:16
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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. –  Will Martin May 10 '11 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 May 10 '11 at 23:12
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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.

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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 May 12 '11 at 13:08
    
+1, this is the perfect way to write that sentence. –  Matthew Read May 12 '11 at 20:32
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