the unit is too small to be measured with practical repeatability using rules alone. (Most rule markings were far too wide to mark a single mil, and even if such dividing is accomplished, it is unclear to the naked eye, being discernible but not useful for measuring.) During the following half century, such measuring instruments, previously expensive rarities, became widespread, everyday tools among machinists. Bringing more metrology into machining made possible, for example, designing an assembly in the form of an engineering drawing, then having the mating parts made at different firms who did not have any contact with (or even awareness of) each other—yet still being sure that their products would have the desired fit. See also Angular mil Circular mil References Imperial units Units of length Decimalisation Customary units of measurement in the United States