In Australia, the most common paper size is A4, and measures 210 x 297 mm. The format from which A4 is derived is a metric system called ISO (International Standards Organisation) 216, a system which uses a ratio of 1 to the square root of 2, yielding a 1:1.414 ratio across all paper sizes. Using this system, when the paper is halved, the resulting size maintains the same proportion. An AO sheet (rounded to the nearest millimetre) measures 840 x 1188 mm, and is 1 square metre. Each ISO paper size is one half of the area of the next size up: A1 is 840 x 594, A2 is 420 x 594, A3 is 420 x 297, A4 is 210 x 297, and so on, all the way to a tiny 26 x 37 A10.