The paper is available in a wide range of standard formats that vary in size. These formats have been established by international standards organisations such as the International Organisation for Standardisation (ISO). The most common standard is the ISO A series, which includes formats A0 to A10. This system is based on the principle that each successive format is half the size of the previous one. For example, A1 is half the size of A0, A2 is half the size of A1, and so on. A4 is the most commonly used format, especially for printing documents and letters. Larger sizes, such as A3, are often used for printing posters or larger documents.