A few years ago when I started learning PCB design for the first time, I encountered a very strange unit. Before used KiCad, I made some of my first PCBs on EasyEDA and something that confused me was what “mil” meant.
I had never heard of a “mil” before, and at the time I didn’t even know it was a real unit. I was slightly confused, but quickly assumed that it was just a weird way of saying millimeters. So when I tried to use those measurements assuming they were in millimeters, I was met with a big surprise.
When I realized the units were definitely off and consulted my search engine of choice, I was shocked to find that a “mil” is a real unit. A “mil” is actually 1/1000th of an inch. Despite being an American, I had never even heard of such a unit. It also definitely doesn’t help at all that the plural form of the unit, “mils”, can very easily be mistaken for millimeters, or even “millions”.
Even the dictionary agrees with me.
In the end, weird and obscure units like this really show how much of a disaster the US imperial system is.