The "A" system of paper sizes is defined in International Standard ISO 216. The largest size, A0, was set so that its area is 1 square metre and all sizes have an aspect ratio of width to length of 1 to the square root of 2 (1.414). Therefore, if an A0 sheet is folded in half across its width, the resultant size is A1 and so on.
The table below summarises the sizes:
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