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When you want to connect two closely related sentences, you can use a semicolon or a dash. (You can also use a dash for other kinds of non-sentential relations). How would you choose whether to use a semicolon or dash?

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up vote 39 down vote accepted

I don't think I was ever taught a clear-cut rule, and as a non-native speaker, I am probably spoiled to some extent by the usage of dashes in other languages. That being said, following nothing but my intuition I would use:

  • a semicolon when the sentences express related, yet independent (especially grammatically independent) thoughts; they could well stand on their own, separated by a period.

  • a dash when the second sentence backs up the first one, nails it down to something, restates or amplifies it, provides reasons or examples, or when the second sentence could not stand on its own "as is" for grammatical reasons.

I will try to demonstrate my point by rewording the notorious examples from The Oatmeal accordingly.

  • My aunt had hairy knuckles; she loved to wash and comb them.
  • My aunt had hairy knuckles — she suffered from hirsutism.

  • When dinosaurs agree on something, they'll often high-five one another; dinosaurs are all about high-fives.
  • When dinosaurs agree on something, they'll often high-five one another — they cannot talk and have to resort to gestures.

  • I gnaw on old car tires; it strengthens my jaw so I'll be better conditioned for bear combat.
  • I gnaw on old car tires — to strengthen my jaw so I'll be better conditioned for bear combat.

Again, this is just my two cents, and I'm only putting them in because the other answers so far seem to miss the point of your question by focusing on non-sentential relations and sometimes not even mentioning semicolons at all. I don't know whether my answer comes close to being correct or not, but I hope it will at least serve as a turning point for getting the discussion back on topic.

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This is exactly the kind of answer I was hoping for. – nohat Oct 14 '10 at 15:51
A semicolon works like a period. A dash works like a colon — or can. That previous use is not one such, though. – tchrist May 31 '12 at 0:52
"I gnaw on old car tires — to strengthen my jaw […]" pretty sure this is grammatically incorrect. – Mk12 Sep 30 '13 at 0:58
@tchrist - If a dash works like a colon, what's the difference between a dash and colon, then? – broofa Sep 7 '15 at 15:16

Semicolon is used to join sentences that can stand alone, but are joined to emphasize their relationship.

En dashes are used to indicate periods of time or other numerical ranges.

Hyphens are used to combine open compounds.

Em dashes are used to disrupt the flow of a sentence and bring emphasis to the coming point. It is a more informal and stronger version of the colon. It can also act as a stronger version of the comma.

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Interesting. Do you have citations for these rules? Are these stylistic rules only? – Mr. Shiny and New 安宇 Aug 26 '10 at 18:53
I am not sure what to define as stylistic and not. Wouldn't that be anything to be used for emphasis? web.mit.edu/comdor/editguide/style-matters/punctuation.html – Eruditass Aug 27 '10 at 3:23
Glad somebody brought up dash types. – Michael Paulukonis Sep 28 '12 at 14:31

Typically, if I'm connecting two closely related sentences, I use a semicolon; I use dashes in instances where commas would also be acceptable—usually where commas would be confusing—or where parentheses would also be acceptable.

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Firstly, you should be aware that there are two different kinds of dashes: the en dash and the em dash.

The em dash (—; historically, an em is precisely the width of the letter "M", but now defined by the height of the font) is generally used to add a passage into the middle or end of a sentence. This is similar to using parenthesis, but should be read without adding a pause. Moreover, the em dash in fact interrupts the sentence, so no pause should be used at all; an interruption should be emphasized.

This dash is also often used as a de facto interruption; for example in dialog when the speaker for some reason cannot continue.

In use, the em dash is not surrounded by spaces. This is done—like this—because it otherwise uses excessive spacing.

Some publishers or style guidelines may instead prefer the narrower en dash (–; an en half the width of an em). This character is usually – as in this example – surrounded by spaces.

Suggestions on the use of semicolons can be found as answers to this question.

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+1 just for mentioning en and em; correcting others' dashes used to be a not insignificant source of grief for me. – Pops Aug 6 '10 at 4:39
@Popular: a much larger problem is the common accidental usage of a hyphen when an em dash is meant. This is worrisome because they have the opposite meaning: hyphens connect and glue things together, and em dashes separate and break things up. It drives me up the wall when a hyphen is used instead of a long dash. It really muddies the sentence up. – Jared Updike Aug 6 '10 at 4:54
@Jared, that grief I referred to was suffered when I was a copy editor, so I know exactly what you mean. – Pops Aug 6 '10 at 5:15
@Jared, I'm slightly worried that incorrect usage of a glyph a few dots or pixels narrower "drives [you] up the wall." Take it easy mate. We're aware of the significance; needn't get angry at those who aren't. Whether it's an en dash, an em dash or a hyphen, it's often quite clear which is intended. – David Foster Aug 6 '10 at 22:34
In American typography it's appropriate to add a thin space on both sides of an em dash, and slightly less commonly, an en dash. Unfortunately thin spaces are difficult to type and sometimes aren't rendered correctly. Still, as someone who's set a lot of type, em spaces pushed right up next to their surrounding words look far too tight to my eyes. Examples with full spaces, thin spaces, and no spaces (pad to push to its own line): M — L vs M — L vs M—L. – Matthew Frederick May 19 '11 at 6:28

In Comma Sense (Richard Lederer and John Shore — ISBN 0-312-34255-1) it's reported that the dash is used:

  • to set off a though or explanatory remark with a sentence: If I remember well — bear in mind I have been on New York City 5 years ago — Central Park is close to Fifth Avenue.
  • to introduce an appositive: When I frequented the ITIS — the secondary school on Cantore Street — I was still a teenager.
  • to signify a sudden change in thought: We are going to — what's that burning smell?
  • before the citation of an author or source of a quotation.

In the other cases, a different punctation should be used.

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protected by Clark Kent May 31 '12 at 18:21

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