I don’t think I’ve ever seen a professionally-set work using spaces around em dashes. The two popular conventions are space–en-dash–space (“a – b”) and em-dash (“a—b”). I only recall ever encountering space–em-dash–space a few times (one of which was within the last few days, and the previous was probably years ago), in amateur work. THIN SPACE would certainly make me much more amenable to it, but frankly that suggests a font problem (and I do certainly know of fonts where the em dash spacing is awful and if forced to use that font I would insert THIN SPACE (or very occasionally NARROW NO-BREAK SPACE) every time).

i think space-em-dash-space was pretty common in both journals and monographs on engineering, science, and math in the 01940s to the 01970s, including some that include really excellent typesetting, though i don't have an excerpt ready to hand to post (and hn isn't really a usable medium for discussing visual things anyway)

i don't remember ever seeing it in fiction

i think of space-en-dash-space as just being an error, and i'm pretty sure i wasn't just misidentifying en dashes with spaces around them as em dashes

> 01940s to the 01970s

Out of interest, why leading zeros?

Might the em dash convention be a byproduct of the printing press and fixed-width layouts?

not in a way that is obvious to me

The spaced em dash occupies 10% more space.

The traditional syntax is particularly suited for newspapers who valued typographic density.

oh, i see what you mean now

but wouldn't that still be the case if you were handwriting your newspaper and xeroxing it or something

where does the printing press come in

also though i don't think fine typography is mostly determined by newspapers

"Em and en refer to units of typographic measurement, not to the letters M and N. (Yes, the homophony is confusing. To disambiguate, loud print shops referred to them as mutton and nut.) In a traditional metal font, the em was the vertical distance from the top of a piece of type to the bottom. The en was half the size of the em. Originally, the width of the em and en dashes corresponded to these units."

https://practicaltypography.com/hyphens-and-dashes.html

Handwriting allows one to vary the point (width) of individual letters. Printing presses do not afford that luxury.

sure, but you can still leave horizontal space around your dashes—or not

what we nowadays call 'microtypography' and think ourselves very avant-garde for employing is ubiquitous in medieval illuminated manuscripts; every line is full of subtle variations in letterforms to better fit the available space