Thou = .001”, also known as a thousandth of an inch, is a specific unit of length based on the inches system of measurement. The thou, sometimes called a mil, is a typical way to measure thickness. Mil used to be more regularly used, but today thou is the more common term. Thou is spelled the same whether referring to a singular or plural amount. Manufacturing and engineering commonly use the thou = .001” as a way to report thickness of objects such as paper, paint layers, sheeting, metals, plastics, and more. In the field of machining, .0001 inches is often called “one tenth” with that really meaning one tenth of a thou. Machinists in particular will measure within a few tenths and consider this level of precision to be satisfactory and accurate. In this way, a thou = .001” is utilized to gauge when a measurement is a high quality one. The introduction of a thou in machining aligned with the increase in usage of screw micrometers and vernier calipers.