Paper 'A' Sizes
The metric standard of ‘A’ sizes of paper starts at ‘A0’, used for large posters, and works downwards in size with each ‘A’ number being half the area of the previous.
This is done by reducing the dimensions by the square root of ‘2’, or dividing by 1.414, to give 71% in size. This system replaced the old imperial measures that came in a whole range of unrelated formats, such as quarto and foolscap used for paper stationery. There are variations within the metric ‘A’ system. For example, ‘C’ and ‘D’ sizes of envelopes are used for holding folded ‘A’ sized paper, and oversized ‘RA’ and ‘SRA’ paper formats are used in the print industry so that bleeds and crop marks can be used, which can then be trimmed back to the standard ‘A’ sizes afterwards.
NOTE: The ‘A4’ format is similar to the ‘letter’ size, used in the United States, but is a little narrower and longer.