Since coming to Japan, I've made a most delightful discovery. At home whenever I created or printed a document on my computer, these mysterious A4, B5, etc. paper sizes always showed up in my choices. What could these cryptic codes possibly mean? How could one ever determine a paper's actual size through a simple combination of letters and numbers? To me, "letter" and "legal" made perfect sense.
I don't know about anywhere else, but in Japan A4 size is the standard "letter" size paper. I assume that much of the rest of the world uses this standard, since in all too many cases it seems to boil down to "the rest of the world" using one standard while the US (and sometimes England) uses some outdated half-assed imperial standard. Note that in no way do I claim knowledge of worldwide paper size standards; I'm talking out my ass.
After an extended period of confusion over these strange paper sizes, it was finally explained to me. Lucky you, that you get to share in my leaned wisdom. Witness: The letter (A,B, etc.) indicates the basic size category of the paper. The number indicates (it seems to me) the relative size in comparison to other numbers in the category. Two combined sheets of one number will always form one sheet of the next number up. For example, two B5 sheets will form, side by side, one B4 sheet. Likewise, two A4 sheets will form a single A3 sheet. Genius!
I imagine this makes for dry reading, but just try for a moment to imagine my excitement when I figured this out. It's like cracking a code or something. It makes the creation of school handouts so much simpler, not to mention cooler because I can do neat things with my handout sheets.
Yet again ... It's the small things in life.