ELI5 Paper Sizes - why does the US use Letter instead of A4?

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. To the community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]