Engineering and machining specifications are usually in 0.001” and referred to as thousandth’s or “thou’s”. Tools like wrenches or drill bits use fractions in the form x+1/2^n like 2 1/2 == 2.5”. It goes down to 1/64, I think.
They also speak of tenths which are 0.0001" units, i.e. "tenth of a thou". This jargon tends to confuse those who are new to it, but has been around for over a century.
