Paper in the A series format has a 1:1.414 aspect ratio(1: square root of 2), though this is rounded to the nearest millimetre. A0 is defined so that it has an area of 1 m², prior to the above mentioned rounding. Successive paper sizes in the series A1, A2, A3, etc., are defined by halving the preceding paper size parallel to its shorter side, again prior to rounding. The most frequently used paper size is A4 (210×297 mm).