Griffith Pioneer Park Museum.
During other work editing documents and learning graphic design software, I was introduced to the differences between a hyphen, an en dash and an em dash.
A hyphen is like a minus sign except it adds words together to create a concept, like air-conditioner.
An en dash is the same size but shows a range, like 9-5 in Dolly Parton's song 'Nine to five' as shorthand for 9am to 5pm business hours.
An em dash is a longer dash and is used as a kind of dynamic in writing, like:
“Familiarity breeds contempt — and children,” Mark Twain once said.
(On another matter, one thing that frustrates me is seeing people use an ellipsis instead of an em dash, as these should signify something is missing from a quotation. So, when I see an advertising slogan like "for the best prices in town... come to our shop," I assume they're saying there's a proviso like "for the best prices in town except the cheaper places that we won't mention...")
Anyway, the difference between en and em dashes these days is less important because digital design software has made a role like typesetter largely redundant. It's one of those professions that computers replaced a long time ago.
However, if you look in the settings of graphic design programs, you can still find evidence of ems as a measurement.