#### Yondering

##### Regular Member

Can you provide a source for that? Or some math explanation how units don't matter. Cause I've never, never, never seen it.

Yes. The math is actually pretty simple: the measurement of a mil is simply a ratio of one "unit" per 1000 of the same "units". If we look at it in meters, we've got "meters per meter", or m/m. Those cancel each other out, and the 1/1000 is unitless; it's a universal ratio that describes a specific angle. Because it uses the same units on both sides of the fraction, it can be used in any measurement system (and it is).

For example, we know that one mil is equal to 3.6 inches per 100 yards, right? That's because 1 mil is 1 yard per 1,000 yards. Converting yards to inches, we have 36 inches per 1000 yards, or 3.6 inches per hundred yards. 0.1 mil is one tenth of that, or 0.36" per hundred yards. If we only measured things in yards and not inches, mils would be used exactly the same way in yards or meters, without the 36 in/yd conversion.

You are correct on the other uses for "mil" such as measuring thickness in machining or manufacturing meaning 1/1000 of an inch, and it's also present in words like "millimeter", meaning 1/1000 of a meter. When you look at the bigger picture, mil is simply a term that means 1/1000, and is not specific to a certain measurement system or type of units, even though it is more commonly used in the metric system.

Does that answer your question? I hope that wasn't too long-winded.