[lolinder]

> The situation with machine tools in the US is unbelievable, by the way. So many stupid mistakes have been caused by the confusion between mils (1/1000 of an inch) and millimeters.

To be fair, this wouldn't be an issue if we had only imperial units. It's only possible to mix these up because we have an awkward mix of both systems.

> The situation is legitimately a little bit different with minutes and hours, since we are able to specify units of time somewhat arbitrarily to match the units, i.e., if hours were 64 minutes instead of 60, many meetings would instead be 64 (or 32) minutes long.

The same situation is true of carpentry, though. when we're building something out of wood we rarely have specific dimensions we must meet, we're building something artificial and can pick the closest round number. 2x4s are 2" by 4" not because they need to be slotted into some naturally-occurring fixture but because it's a nice round number that is sufficiently useful. The standard ceiling height in the US is 8' not because of any law of the universe but because it's a round number that's easy to measure and easy to divide (quarters, halves).

Your examples of something 7.3 feet long or 7.87 feet long simply don't come up in carpentry because we'd usually just round up to a nice number and scale the whole project accordingly. And if we do have something that needs to be precise within +/- a certain tolerance, a base unit that is easily divisible is more likely to be able to round conveniently while remaining within the tolerance.

[kelnos]

Ugh, so I was just making some new window screen frames to replace some that were missing, and what a pain that was to measure things out. I had fun dimensions like 21-5/16" all over the place. Now, they certainly didn't need to be accurate to a sixteenth-inch, but being off by even a quarter inch could easily make them not fit.

Too large and you just can't get the screen into the frame, and too small and it can just fall out, or at the very least leave gaps large enough that bugs can get through.

I agree with you that there are many things where we don't have a specific physical need for certain dimensions, so we just pick round numbers. But I think the cases where we do have to conform to some messy existing physical dimensions come up more frequently than you think.