“Breaking Paragraphs into Lines § A Historical Summary”, 1981-11 (; similar):
…We have now discussed most of the issues that arise in line breaking, and it is interesting to compare the newfangled approaches to what printers have actually been doing through the years. Medieval scribes, who prepared beautiful manuscripts by hand before the days of printing, were generally careful to break lines so that the right-hand margins would be nearly straight, and this practice was continued by the early printers. Indeed, printers had to fill up each line of type with spaces anyway, so that the individual letters wouldn’t fall out of position while making impressions, and it wasn’t too much more difficult for a compositor to distribute the spaces between words instead of putting them at the ends of lines.
One of the most difficult challenges faced by printers over the years has been the typesetting of ‘polyglot Bibles’—editions of the Bible in which the original languages are set side by side with various translations—since special care is needed to keep the versions of various languages synchronized with each other. Furthermore the fact that several languages appear on each page means that the texts tend to be set with narrower columns than usual; this, together with the fact that one dare not alter the sacred words, makes the line-breaking problem especially difficult. We can get a good idea of the early printers’ approaches to line breaking by examining their polyglot Bibles carefully.
[Knuth & Plass discuss instances like the Complutensian Polyglot Bible: a page might have up to 6 languages, all in different scripts with varying line lengths but intended to be cross-referenced for comparison to allow scholarly study and check accuracy. The technical demand was solved using hyphenation & justification, and spaces might be filled with ‘ooo’ to give solid blocks of text. The justification was manual, often somewhat sloppy and loose, clearly done one line at a time, with resort to abbreviations and irregular spacing of punctuation to assist. While the TeX Knuth-Plass algorithm described in this paper may never be able to match 1926 The Art of Spacing for perfection, Knuth feels it is better than most human typesetters can afford to do, even on the most prestigious and well-funded projects like the polyglot Bibles.]