One of the greatest advances in the art of printing was the invention of movable type, for this permitted the printer to be free of the time-consuming process of having his plates individually engraved by hand.
Yet movable type, too, had disadvantages, for it had to be carefully adjusted for even depth of impression and, once set up, had to be firmly locked in place to avoid becoming “pied” (i.e., scrambled).
Then there came another great advance, said to be the invention of Firmin Didot, a French printer, in about 1798.
This was stereotype, from the Greek stereos, “solid,” plus typos, “type” (from typtos, “to strike”); hence, “solid type.”
In this process, movable type is set, as before, but it is then used to form a mold which, in turn, is used to cast a solid plate of type, releasing the original type for reuse and avoiding pieing.
But one of the greatest advantages to the printer was that the invention of stereotype permitted the running of much larger editions, at lower cost, than had theretofore been possible, and each copy assuredly being exactly like all its mates.
This absolute duplication of copies led to the figurative use of the word to describe people who behave in uniform patterns, also to characterize hackneyed phrases or those who use them, hence any formalized or uniform pattern of behavior.