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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.
As your links explain, C series is used for envelopes, while B and elongated A are special case that aren't commonly used.
In any case, none of those series has an equivalent to American Letter.
The only paper that you will commonly find in European offices are A4 and (to a lesser extent) A3.
A big office printer might have 4 trays stocked with A4 and one tray with A3, for example.