The ideas above developed gradually, first in the armory practice of the mid and late nineteenth century (otherwise known as the American system), and then in true mass production during the twentieth century. Those two phenomena have not always been differentiated from each other, but the difference is in the degree to which toolpath control had replaced skilled fitting, or, as it is more often expressed, the degree to which "the skill had been built into the machine tool". The replacement did not happen overnight, but rather was a gradual tapering off of reliance upon fitting, the progress varying by plant and by decade, until it had been completely eliminated from the assembly process, creating true mass production.