ELI5 Paper Sizes - why paper sizes in the US are different?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

What’s the historical reason for the U.S. using "Letter" paper size over the international standard A4 paper? What inconveniences do people face in North America due to the different paper sizes compared to the rest of the world? How does the unique aspect ratio of A4 paper help in scaling paper without wasting space? What about B and C format paper?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: jizzwaffle, chuggerbomb, svargas05, avgjoe33, jackwell, and bizzareusername28.

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