The old linotype machines had visual indicators of minimum and maximum line width, and the operator would make a judgement call with each line. Spacers would t

The old linotype machines had visual indicators of minimum and maximum line width, and the operator would make a judgement call with each line. Spacers would then automatically justify the letters. It was all mechanical and amazing. See http://widespacer.blogspot.com/2014/01/two-spaces-old-typists-habit.html http://widespacer.blogspot.com/2014/01/two-spaces-old-typist... for many details.