Comment by inkyoto

Before the wide adoption of Unicode in mainstream operating systems, quite a few people used -- (two ASCII minus signs) to differentiate between a hyphen and a dash (of either pedigree), and some people used -- in emails and online where a dash was required.

Most think that it came from TeX, which had -- (for an en dash) and --- (for an em dash, although I don't think I have ever observed it out in the wild outside TeX), but in fact, the habit well predates TeX and goes all the way back to typewriters where typists habitually hit two hyphens in a row to approximate an em dash. The approximated em dash was described in hard-copy manuscript preparation rules such as The Chicago Manual of Style.

So, if you have ever used a typewriter or TeX, you can claim an even richer than 20 years’ heritage of using the em dash.