Deutsches Institut für Normung
DIN 476: international paper sizes (now ISO 216 or DIN EN ISO 216). Specifies the System where the paper size DIN A4 is from. In short, there are different series (A,B,C,..). Each series has a number of different sizes. If you go up one with the number, you halve the size. DIN A3 is as large as two pages of DIN A4. The biggest in that series is DIN A0. See Paper size for more.