The International Organization for Standardization Paper Sizes
European standard paper size standard. The most common of which is the ‘A’ series.
The aspect ratio of ISO paper sheets is 1 to 1.414.
This means that if you cut a sheet into halves they will be the same proportion as the original. In other words a sheet of A3, when cut in half, will give two sheets of A4.
There are are also B and C size papers and the larger RA and SRA paper sizes, which are allow printers to print oversize sheets that can be trimmed to ‘A’ size for commercial use.