Paper size refers to the size (length x height) of a sheet of paper. The international paper size standard, ISO 216, is based on the German standard DIN 476. The ISO sizes are all based on a single aspect ratio of the square root of 2. The basic size is a 1 m² sheet of paper (A0). Later formats (A1, A2, A3 and so on) are obtained by simply cutting the paper in half on the longer side. The most used and well-known format is A4 (210 × 297 mm). In addition to the A series, there is a less common B series in conjunction with a C series. The area of ​​the sheets in the B series is a geometric mean of the A sheets while the C series is used almost exclusively for envelopes and is defined in the standard ISO 269.