How to Use Em Dashes (–), En Dashes (–), and Hyphens (-)

* Hyphens connect things, such as compound words: double-decker, cut-and-dried, 212-555-5555.

* EN dashes make a range between things: Boston–San Francisco flight, 10–20 years: both connect not only the endpoints, but define that all the space between is included. (Compare the last usage with the phone number example under Hyphens.)

* EM dashes break things, such as sentences or thoughts: 'What the—!'; A paragraph should express one idea—but rules are made to be broken.

Unicode has the original ASCII hyphen-minus (U+002d), as well as a dedicated hyphen (U+2010), other functional hyphens such as soft and non-breaking hyphens, and a dedicated minus sign (U+2212), and some variations of minus such as subscript, superscript, etc.

There's also the figure dash "‒" (U+2012), essentally a hyphen-minus that's the same width as numbers and used aesthetically for typsetting, afaik. And don't overlook two-em-dashes "⸺" and three-em-dashes "⸻" and horizontal bars "―", the latter used like quotation marks!

Some style guides recommend "space, en dash, space" for this, and I prefer that myself – mainly because some software doesn't treat em dashes correctly as word separators for double click selection purposes.

For example, I'm pretty sure that at least some Kindle models would highlight both the word before and after the em dash when selecting one of them, which makes using the dictionary very annoying.

(I also now looked up and found out that in Spanish, apparently, you are supposed to put space only on one side of the dash, when used as a direct speech separator.)

For what it's worth, I was in the last class in my high school to learn typing on IBM Selectric typewriters. We were taught to type two spaces, two hyphens, then two spaces. Incidentally, we were taught two spaces after periods and colons. To this day, I find it hard to read text that doesn't have proper spacing after periods. (HTML and WYSIWYG word processors handle formatting, but e.g. fixed-font text editors don't)