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It blows our hivemind that the United States doesn't use the ISO 216 paper size standard (A4, A5 and the gang).
Like, we consider ourselves worldly people and are aware of America's little idiosyncrasies like mass incarceration, the widespread availability of assault weapons and not being able to transfer money via your banking app, but come on - look how absolutely great it is to be European:
The American mind cannot comprehend this diagram
[Diagram of paper sizes as listed below]
ISO 216 A series papers formats
AO
A1
A3
A5
A7
A6
Et.
A4
Instead, Americans prostrate themselves to bizarrely-named paper types of seemingly random size: Letter, Legal, Tabloid (Ledger) and all other types of sordid nonsense. We're not even going to include a picture because this is a family-friendly finance blog.
The US officially uses the metric system...to define the US customary system of measurements.
E.g. 1 foot is defined as 0.3048 meters.
That was just ONE aspect of the adoption of the metric system. And that was done to make backwards compatibility easier because of long lasting legacy systems. Because some systems, like say plumbing in a home or city, last for a very long time before they get replaced or repaired. New plumbing installations often use Tubing like PEX which is metric. I designed and fabricated a dozen brackets for a US customer over the weekend for delivery today. They were made using millimeters as specified.
And if you actually pay attention to how the metric systems compares to the US customary, you would see just how closely they are related from the start. Which makes sense since what we call Imperial measurements predate the metric system by several hundred years.
Plumbing is still described in customary in Europe. You have 1/2 or 3/4 fittings etc.
Imperial is actually just a couple years older than US customary, it was predated by the "English units".
https://en.wikipedia.org/wiki/English_units
You are correct. But Imperial is pretty much abandoned and the US only kind of sort of followed it. There have always been some differences between US Customary and Imperial. The gallon is one of the most obvious differences.