Papers manufactured outside North America are based on a 1 square meter measure. It is both the basis for the grams per square meter (gsm or gm2) measurement of weight and the exact area of an A0 sheet of paper. The A series of sizing is the most common sizing system for stationery. As the sizes progress in the series, each sheet size is twice the size of the smaller and half the size of the next larger.
Although the A series of measurement is an International Standards Organization (ISO) system of paper sizes, occasionally A size labeling does not conform to ISO sizing standards. It is always best to double check your measurements when purchasing A series stationery.