punctuation - Should I use a semicolon or a dash to connect two closely related sentences?

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?

Answer

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.