We have four kinds of horizontal lines in English typography. Everybody knows about the hyphen; you even have two keys for it on your keyboard, the minus key, and up toward the right end of the top row of keys. Speaking of that key, the slightly longer horizontal line above the hyphen is, counterintuitively, the underscore character. (If you want the underscore under letters, you have to use the underscore font style, Ctrl-u in MS Word.)
You might or might not know about the other two horizontal lines, the N-dash and the M-dash. (Alt-0150 and Alt-0151 respectively. Hold down the Alt key while you type the digits on the numeric keypad.)
- Use the N-dash to show a range; your work hours are 9–5, for example.
- Use the M-dash to show a break in thought. It’s like a strong parenthesis.
And here we come to the matter of style:
Don’t put spaces around your dashes.
Those spaces waste space. Here’s an otherwise good sentence with those bad spaces:
That year — 2014 — three young quantum gravity researchers came to an astonishing realization.
https://www.quantamagazine.org/how-space-and-time-could-be-a-quantum-error-correcting-code-20190103
Yes, you can insert the spaces, but don’t.
PS— I just ran into an alternative to the M-dash in a place where I’m not used to seeing it: Professional writing. That alternative is two hyphens. Typing two hyphens is okay for casual writing, say, on a typewriter, but not in an ezine article. I suspect his editor was asleep n the job. Here’s the sentence:
Hope you stayed up late watching West Coast basketball (and/or the Masked Singer premiere) last night — otherwise you might’ve missed the quasi-surprise drop of this April’s entire Coachella lineup at 11:28 p.m. ET.
https://www.billboard.com/articles/news/festivals/8492113/coachella-2018-lineup-five-takeaways
Don’t do that, either.