When linotype machines were in use, or even earlier when type was set by hand (anyone remember the California job case?), you had to put in spacers to make all the lines come out the same length. There would be spacers between words or letters, and also between lines and paragraphs, for horizontal and vertical justification, so that the type would one rectangular block that could be set into the printing press.
You can still get letterpress type hand and machine set. There are specialist firms that do it. The main customers are people who want that "printed" look, blurs and overimpression and all.
I was using a California case as late as 1975. Actually, it referred to the layout of the type within the case rather than the case itself, but hey, who's worried? You had three basic space units: Ems, ens and thins. The Em was the same width as a capital M in the font being used. The En was half the width of the Em and the thin was actually an abritrary width. We also had variations around the En - thin area. There were quite strict rules about handset justification which spilled over into machineset justification. All thrown out with the advent of the word processor, of course. Most printing, in all areas, is absolute rubbish today in terms of quality, but even I'm beginning not to notice any more. It's not a losing battle, it's been lost, game, set and match.
