I guess around fourteen years ago—back when I was at high school—I picked up the legendary TeXbook. I was in love and totally beguiled by the great Knuth's Art of Computer Programming; how could it be so beautiful? how could it be so precise? and henceforth as I understood the he had developed that masterwork of TeX to make his book, I immediately went to read it. There, in page four he writes:
I guess around fourteen years ago—back when I was at high school—I picked up the legendary TeXbook. I was in love and totally beguiled by the great Knuth's Art of Computer Programming; how could it be so beautiful? how could it be so precise? and henceforth as I understood that he had developed that masterwork of TeX to make his book, I immediately went to read it. There, in page four he writes:
> Book printing differs significantly from ordinary typing with respect to
> dashes, hyphens, and minus signs. In good math books, these symbols are all
> different; in fact there usually are at least four different symbols:
>
> a hyphen (-);
> an en-dash (–);
> an em-dash (—);
> a minus sign (−).
>
> Hyphens are used for compound words like ‘daughter-in-law’ and ‘X-rated’. En-
> dashes are used for number ranges like ‘pages 13–34’, and also in contexts like
> ‘exercise 1.2.6–52’. Em-dashes are used for punctuation in sentences—they are
> what we often call simply dashes. And minus signs are used in formulas. A
> conscientious user of T E X will be careful to distinguish these four usages, and
> here is how to do it:
>
> for a hyphen, type a hyphen (-);
> for an en-dash, type two hyphens (--);
> for an em-dash, type three hyphens (---);
> for a minus sign, type a hyphen in mathematics mode ($-$).
So I kept using the beautiful em dash ever since; because that is how the greats do it.