The most annoying thing about the thousandth of an inch is while it’s often called a ‘thou’, it’s also called a ‘mil’ which sounds exactly as the common slang for millimetre (mill) here in Australia at least (used more often in construction, but it’s still common so makes it confusing).

Luckily we don’t really do PCB design in imperial anymore at any of the companies I’ve worked with (I hear even US companies are moving away from it since so much of the manufacturing is offshore)