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I am considering how to choose between em dashes and double commas. The example I am considering is a sentence originally of the form

We will give another proof of a theorem in 'some' book, an assertion 'such' and 'such', using 'this' approach; the approach is 'such' and 'such'.

For me a sentence like this one is quite clear. But somehow a thought suggested that perhaps there are two many nonperiods in a sentence? Readability will be suppressed? Then I realized replacing the two commas with em dashes seem to preserve the same meaning and potentially improve the readability:

We will give another proof of a theorem in 'some' book -- an assertion 'such' and 'such' -- using 'this' approach -- the approach is 'such' and 'such'.

I am not sure. I would like to have a rule of thumb.

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    Overall your fake/example sentence doesn't read very well, and I don't really understand your use of single-quotation marks, so it's rather hard to say whether your sentence justifies the use of either em dashes or commas. Also, it seems you're overusing the em dashes there. The sentence is confusing as is. Better to focus on clarity of syntax than on punctuation, generally speaking. – Tᴚoɯɐuo Dec 31 '18 at 12:31
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Generally speaking, yes, em dashes do improve readablity.

Em dashes in place of commas - A pair of em dashes can be used in place of commas to enhance readability. Note, however, that dashes are always more emphatic than commas.

And yet, when the car was finally delivered — nearly three months after it was ordered — she decided she no longer wanted it, leaving the dealer with an oddly equipped car that would be difficult to sell.

I would suggest using em dashes ( — ) instead of commas ( , )

  • I've downvoted because you provide no good reasons for your preference. Your answer is basically circular logic -- you say the em dash is better because it improves readability, but it's more readable because you think it's better. Moreover you fail to address some of the grammatical problems in Gary Moore's example. – Andrew Feb 14 at 8:16
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Although this is not from a style guide, I agree with Jim Davis' opinion on Quora:

In general, I don’t recommend using em dashes in academic writing, particularly in APA style. In scholarly writing, ideas should be expressed in as linear an order as possible. Because em dashes are used for insertion of information in a non-linear way, that is, out of order, there is probably a better, more linear way to express it. If you must insert information out of order, use parentheses.

For a more definitive opinion, you should consult a style guide from a reputable institution.

In any case, if you must use the em dash then there should be no more than one pair in a sentence, and it should contain significant enough information to merit special treatment. If it's something that's only marginally relevant or of little interest, consider leaving it out.

Lastly there should never be three em dashes in a single sentence. An em dash should begin and end the parenthetical statement, and appear nowhere else.

In your example given it's not clear that the statement "an assertion 'such' and 'such'" merits a parenthetical assertion. It seems like important information that should be an essential part of the sentence, and not something like an expansion on something you've already said. So I see no reason to use the em dash.

Perhaps if you posted an example with the actual words you plan to write, rather than placeholders, I could give you a more informed opinion.

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