>They specify dimensions in feet, inches, and fractions of an inch. But not all of them do.
There are many more fun and exciting non-metric measurements you might encounter than plain old fractional inches.
A fabricator might encounter sheet metal thickness in "gauge". Wire sizes, ammunition, and machine screws also come in "gauge" sizes but all four are different scales. US drills come not only in fractional inch sizes, but letters and numbers as well. Furnace efficiency is often specified in percent, but air conditioner efficiency comes in SEER. Water softener capacity is in "grains". Pipe threads come in "inch sizes", but that usually means NPT.
Metal hardness and rubber durometer measurements have their own scale which doesn't really belong to either camp.
To be fair, a lot of these are categorical units. Screws come in #2 or #6 or #4, but you'll never need to worry about #3.7.
A wise professor once told me "All these different units will not be going away within your lifetime, so you better get used to working with them."
Gauges are a measurement, but they aren't a unit. They don't necessarily have any linear correspondence to actual distance/volume/whatever, and in fact are typically inverse (higher number = smaller thing). And yes, they're categorical. (Sometimes you even see 00 as a gauge number.)
Metal hardness having a bunch of different units is quite annoying but also interesting.
There are quite a few ways to measure the hardness the most interesting being Vickers. You plunge a diamond of a known force into a surface and measure the size of the indentation. This is surprisingly accurate but does leave a small diamond mark on your surface.
With a sufficiently small part a blacksmith or other folks can determine the hardness of a steel just by listening to how it rings. Hey, you can also test for cracks with a ring test, the most common use is ring-testing a vitrified grinding wheel to see if there is no crack
Alot of engineering is just listen and maybe the odd hit with a hammer
There are many more fun and exciting non-metric measurements you might encounter than plain old fractional inches.
A fabricator might encounter sheet metal thickness in "gauge". Wire sizes, ammunition, and machine screws also come in "gauge" sizes but all four are different scales. US drills come not only in fractional inch sizes, but letters and numbers as well. Furnace efficiency is often specified in percent, but air conditioner efficiency comes in SEER. Water softener capacity is in "grains". Pipe threads come in "inch sizes", but that usually means NPT. Metal hardness and rubber durometer measurements have their own scale which doesn't really belong to either camp.
To be fair, a lot of these are categorical units. Screws come in #2 or #6 or #4, but you'll never need to worry about #3.7.
A wise professor once told me "All these different units will not be going away within your lifetime, so you better get used to working with them."