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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.
Letter paper (8.5" x 11" | 215.9mm x 279.4mm) is kinda sorta pretty close to A4 (8.27" x 11.69" | 210mm x 297mm) so without having the two next to each other, it can seem like A4 is just a funny piece of letter, and vice versa. But to answer the actual question, USA and Canada (and apparently the phillipines???) use the "North American Standard" which is a terrifying mess in comparison to the beauty that is the ISO standard.
Edit: typos