I learned recently about the standard A-series paper sizes (ISO 216) and was really impressed. I don’t know why these standards aren’t used everywhere, they’re so perfect.
Take a sheet of paper with an area of one square meter and a width-length ratio of √2 and call it A0. Cut that sheet in half across the longer side and call it A1. The new sheet is exaclty half the size of the first sheet and has the same width-length ratio of √2. Amazing.
Take your regular A4 sheet (similar to 8.5″×11″) to the copier, shrink it by 50% and it fits perfectly on an A5 sheet. Terrific!
It’s like a beautiful, rectangular golden spiral. I love it.