I am currently writing a formal piece of academic work in which I want to add a supplementary, parenthetical clause in the middle of another sentence. However, the supplemenatary clause is itself a stand-alone sentence. I don't want to bog you down with the technical guff from my work and so will illustrate with a different example:

Not only are penguins flightless birds [pause They are. pause], they are also aquatic.

Note that the example above is not a suggested format for the sentence (I am deliberately trying not to beg the question).

My question is:

  • How should I punctuate the bolded parenthetical phrase in the sentence above?

I am not asking about how to reformulate the sentence to avoid thorny punctuation issues. I would like to represent this sentence exactly how I might say it in a formal presentation without the addition of any ands or other reformulations.

My concerns are:

  1. I want to emphatically assert in my parenthetical phrase that penguins are flightless.

  2. The parenthetical phrase is effectively a stand-alone sentence inside another sentence. So I am unsure about whether the first word should/can be capitalised.

  3. I am unsure about any punctuation at the end of the clause before the first, larger sentence recommences.

  4. I'm unsure about any encompasing punctuation devices I could use before and after the phrase (in conjunction with the other issues mentioned in (1-3) above.

Of course, I could easily say and they are. However, that is not the sentence I'm trying to represent in the writing. Please do not recommend the and. I'm not interested. I want the words to be exactly and only the words used in my example.

  • Comments are not for extended discussion; this conversation has been moved to chat. – tchrist May 11 '17 at 17:27

As I understand it, you want to put an entire sentence--or more than one--within another. I'd put it in parentheses, following the closing parentheses with whatever punctuation you would have used before the first paren. I would hope you would not be using other parentheses in the sentence.

The other option is square brackets. I would consider brackets only for unrelated instructions [see sidebar] to the reader.

I would not, as a matter of style (it would confuse your reader), start the internal sentence with a capital; and probably would not put a period (a period would cause your reader to pause, and the parentheses are enough) at the end.

Thus: Not only are penguins flightless birds (they are), they are also aquatic.

Consider: Not only are penguins flightless birds (I have seen them struggling on the ice), they are also aquatic (this too I have seen, as they plunge into and scramble out of the frigid water).


Chicago Manual of Style 16th Edition (on-line but behind paywall)

6.92 Use of parentheses
Parentheses—stronger than a comma and similar to the dash—are used to set off > material from the surrounding text. Like dashes but unlike commas, parentheses can set off text that has no grammatical relationship to the rest of the sentence.

The disagreement between Johns and Evans (its origins have been discussed elsewhere) ultimately destroyed the organization.

From my hard copy of the Chicago Manual of Style, 15th edition, 6.103:

A question mark, an exclamation point, and closing quotation marks precede a closing parenthesis if they belong to the parenthetical matter; they follow it if they belong to the surrounding sentence. A parenthetical enclosure of more than one sentence should not be included within another sentence.

Joan Didion's The White Album (essays) uses parentheses in this way often; she is considered an excellent stylist.

These references confirm the examples and statements in this answer--just thought you'd like sources.


It's certainly a matter of style, and, depending on who the audience is, your choice may be important.

From the Pocket Writing and Style Guide of the U.S. Naval War College, page 12:

Dash: used for emphasis or to represent a sudden break in thought; frequently used when a writer wishes to represent linguistic patterns in dialogue (e.g., repetition, afterthoughts). Use sparingly, particularly in academic writing. Correct use of Dash:

 To point out a sudden break in thought or set off a parenthetical element. On War—a seminal text on the theory of war—is as relevant now as it was in the 19th century.


Incorrect use of Dash: In scholarly writing if another punctuation mark can be used in its place.

I’m running three races this month—a 5k, a 10k, and a half marathon. Corrected: I’m running three races this month: a 5k, a 10k, and a half marathon.

  • OP is not asking about the choice between dashes, brackets, commas or zero punctuation to offset here (and this would have made the question a duplicate). – Edwin Ashworth May 2 '17 at 23:16
  • If he is not, then it's not clear to me what he is asking. My comment at the question was only partly tongue in cheek. Some elaboration and showing of research would be helpful. It may even have unearthed the duplicate. I believe you are probably right that this has been covered -- more than once, no doubt. – Canis Lupus May 2 '17 at 23:21
  • It's because the bit that would be set off by dashes is a sentence in its own right. – Araucaria May 2 '17 at 23:23
  • 1
    I'm not aware of any style guideline that limits parentheticals to sentence fragments – Canis Lupus May 2 '17 at 23:27
  • Right. So the punctuation for sentences and parentheticals is different in most cases. But there is one set of rules for sentences (initial capital letter, etc), and another for parenthetical supplements (no capital letter-becaus a capital signals a new sentence). – Araucaria May 2 '17 at 23:44

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