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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?
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.
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.
May 10, 2011 at 22:28
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.
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.
