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?
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.
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.
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.
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.
Here is what I think:
Semicolon is a mood-switcher
It switches your "learning mood" to "exciting mood" and vice versa.
When dinosaurs agree on something, they often high-five one another; dinosaurs are all about high-fives.
Godzilla is a misunderstood creature; beneath his raging desire to set people on fire & eat them lies a gentle giant who just wants to cuddle
Dash is an explainerIt further explains a concept before it.
When dinosaurs agree on something, they'll often high-five one another — they cannot talk and have to resort to gestures.
Godzilla is a misunderstood creature — beneath his raging desire to set people on fire & eat them lies a gentle giant who just wants to cuddle
This answer is inspired by RegDwigнt's answer.