What is a thou?

A thou, also known as a thousandth of an inch, is a unit of measurement used in engineering to refer to a very small distance. It is equal to 0.0001 inches or 0.00254 millimeters. A thou is often used to express the tolerance or precision of mechanical parts, such as the clearance between two mating components or the diameter of a drill bit or wire. The use of a thou requires precise measurement equipment, such as a micrometer or dial caliper, as even a slight variation in measurement can result in significant differences in the performance of the part or system.