The Gutenberg quandary
But Gutenberg had a problem. The system he devised for assembling type into a page required that lines be of even length. That is, he had to set what we now call justified type. If you look at images of Medieval illuminated manuscripts, the high art of the monastic scribes, you can see that lines end where they end, roughly in even columns, but not precisely so. But the simple fact of placing metal blocks into a frame and locking them in place for printing requires justification (occasionally Gutenberg used a spacing quad at the end of a line, too, something we would not generally do today in justified text).
Breaking words at syllable boundaries was not something that necessarily occurred to Gutenberg. It was more important, with the textura letter style in which his first fonts were cut, that word spaces be consistent, so as to give the page what typographers call an even color. (Textura is so called because the page resembles woven textile. The words texture and text come from the same root.) Yet he did want to indicate somehow that words had been broken. To do so, he used a quill pen to make a hyphen to the right of any broken word. The calligraphic hyphen in use by scribes of the day was a pair of slanted short lines. You can zoom in on this image to see the variation from line to line, evidence that these hyphens were rendered by hand. Considering the many hours of hand illumination that went into each page of the Mainz 42-line Bible after it was printed, a few seconds to toss in the hyphens was not a major expense.
[An aside: Those illuminations were based on the manuscript known as the Göttingen Model Book, which provides step-by-step instructions for rendering them.]
Now if you look at the images of the 42-line Bible, you can see that the hyphens are not actually within the justified type column. Of necessity, they hang to the right of the column. It’s my hypothesis that it is precisely this practice, Gutenberg’s practical accommodation to the quandary of word division, that is the historical basis of the “hanging punctuation” that has been a desideratum of fine typography for at least the last hundred or so years.
Gutenberg’s slanted textura hyphen has also survived to this day as the standard proofreader’s mark for a hyphen to be inserted in composed text.
But you already knew all that, right?