Markus Kuhn (latterly from Uttenreuth!) explains the ISO paper system (A4 etc.) – via Boing Boing. Window envelopes are not standardized.

There are interesting notes on the history of paper sizes. The DIN formats were adopted in Germany in 1922, and in Britain in 1959 (but I can remember using foolscap and quarto long after that).

bq. The United States and Canada are today the only industrialized nations in which the ISO standard paper sizes are not yet widely used. In U.S. office applications, the paper formats “Letter” (216 × 279 mm), “Legal” (216 × 356 mm), “Executive” (190 × 254 mm), and “Ledger/Tabloid” (279 × 432 mm) are widely used today. … The “Letter”, “Legal”, “Tabloid”, and other formats (although not these names) are defined in the American National Standard ANSI X3.151-1987. …

bq. Using standard paper sizes saves money and makes life simpler in many applications. For example, if all scientific journals used only ISO formats, then libraries would have to buy only very few different sizes for the binders. Shelves can be designed such that standard formats will fit in exactly without too much wasted shelf volume. The ISO formats are used for surprisingly many things besides office paper: the German citizen ID card has format A7, both the European Union and the U.S. (!) passport have format B7, and library microfiches have format A6. In some countries (e.g., Germany) even many brands of toilet paper have format A6.

There is advice for North Americans (can they get A4 yellow legal pads?)