“The Monotype 4-Line System for Setting Mathematics”, 2007-08-13 (; similar):
[Author blog. Description of the most advanced mechanical typesetting system for the challenging task of typesetting mathematics (which high-quality typography is what Knuth aimed to recreate).
To provide the typographic quality of hand-set math but at an affordable cost, the Monotype corporation made a huge investment post-WWII into enhancing its mechanical hot metal typesetting system into one which would encode every mathematical equation into symbols placed on a vertical grid of 4 horizontal ‘lines’, into which could be slotted entries from a vast new family of fonts & symbols, all tweaked to fit in various positions, which would then be spat out by the machine into a single solid lead piece which could be combined with the rest to form a single page.
This allowed a skilled operator to rapidly ‘type’ his way through a page of math to yield a beautiful custom output without endlessly hand-arranging lots of little metal bits.]