Use of the dashes is for information not integral to the sentence or for emphasis.

The first example below feels as if it has to be set off from the rest of the sentence as apposed to the other example which sounds correct both ways.

The thought of it – the anniversary – still manages to twist the knife.


The thought of it the anniversary still manages to twist the knife.

Is this dash optional? What about this example?

The president – and his assistant – traveled by private jet.

as opposed to:

The President and his assistant traveled by private jet.

Is there a term for when the element set off from the sentence requires it as opposed to it being optional?

  • There's a technical term for your first example, which I can't remember or easily look up. It's a special case where the subject (it) is "parenthetically" repeated / restated / referred to using a different expression (the anniversary). Any such parenthetical "optional addition" pretty much has to be orthographically set off (with dashes, commas, or whatever). It's not obviously the same in your second example, where that "setting off" is itself optional (if the speaker is only including the assistant as an "afterthought", set it off, otherwise don't). – FumbleFingers Reinstate Monica May 30 '18 at 14:29
  • Please note: you have only provided examples of the use of the dash, not parentheses, so maybe you can take that out? Or put in an example of it. – Lambie May 30 '18 at 14:32
  • In your first example, the second sentence would not contain the word "it": The thought of the anniversary still manages to twist the knife. You only need the dashes because the word "it" is there: The thought of it – the anniversary – still manages to twist the knife. – J.R. May 30 '18 at 14:50
  • There's a related question about dashes on ELU. – J.R. May 30 '18 at 15:03

I think you're looking for parenthetical. A parenthetical statement or clause is one that is not essential to the main sentence (i.e. it can be contained in parentheses). The opposite would be non-parenthetical or some synonym of essential.

Dashes and parentheses mostly work the same. Parentheses whisper while dashes shout. Commas are somewhat in-between. All three can be used to set off parenthetical statements.

Both of your examples are parenthetical statements. If you remove them from the sentence, it does not significantly change the core meaning or intent. The only difference in the following sentences is the tone:

The president (and his assistant) traveled by private jet.

The president -- and his assistant -- traveled by private jet.

The president, and his assistant, traveled by private jet.

If the fact that the assistant also traveled is essential, then it's not parenthetical, and should not be contained in one of these parenthetical structures:

The president and his assistant always travel together by private jet.

You may also be thinking of restrictive and non-restrictive clauses. Non-restrictive clauses are bonus information that can usually be enclosed in some kind of parenthetical structure.

Kaylee (who just graduated from high school) is an accomplished figure skater.

Side note: Pairs of dashes are used to set apart parenthetical information. A single dash is a different animal, and is used to highlight a dramatic action or statement.

They saw them too late as their ship rounded the headland -- pirates!

  • Passing the Bar exam—that was his goal. – bluebell1 May 31 '18 at 22:17
  • Single dash : only statements and dramatic actions. Not full sentences. Say you wanted to add a sentence as extra information for instance. – bluebell1 May 31 '18 at 22:21

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