versus thou In the United States, mil was once the more common term, but as use of the metric system has become more common, thou has replaced mil among most technical users to . Today both terms are used, but in specific contexts one is traditionally preferred over the other. Equivalents in other units of length 1 thou is equal to: 0.001 international inches (1 international inch is equal to 1,000 thou) 0.0254 mm, or 25.4 μm (1 millimetre is about 39.37 thou) History of usage The introduction of the thousandth of an inch as a base unit in engineering and machining is generally attributed to Joseph Whitworth who wrote in 1857: ... instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths ... Whitworth's main point was to advocate decimalization in place of fractions based on successive halving; but in mentioning thousandths, he was also broaching the idea of a finer division than had been used previously. Until then, workers such as millwrights, boilermakers, and machinists in the Anglosphere measured only in traditional fractions of an inch, divided via successive halving, usually only as far as 64ths (1, , , , , , ). Each 64th is about 16 thou. Communication about sizes smaller than a 64th of an inch was subjective and hampered by a degree of ineffability—while phrases such as "scant 64th" or "heavy 64th" were used, they were imprecise. Dimensions and geometry could be controlled to high accuracy, but this was done by comparative methods: comparison against templates or other gauges, feeling the degree of drag of calipers, or simply repeatably cutting, relying on the positioning consistency of jigs, fixtures, and machine slides. Such work could only be done in craft fashion: on-site, by feel, rather than at a distance working from drawings and written notes. Although measurement was certainly a part of the daily routine, the highest-precision aspects of the work were achieved by feel or by gauge, not by measuring (as in determining counts of units). This in turn limited the kinds of process designs that could work, because they limited the degree of separation of concerns that could occur. The introduction of the thou as a base unit for machining work required the dissemination of vernier calipers and screw micrometers throughout the trade, as