C0

The C0 paper size has dimensions of 917 x 1297 mm, making it the largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.99 m2, C0 envelopes are quite large and not commonly used. The C series sizes are geometric means between the A and B series sheets, so C0 lies between A0 (841 x 1189 mm) and B0 (1000 x 1414 mm). While not a typical envelope size, C0 can be useful for oversized and specialty mailing needs. Its origins trace back to efforts in Germany in the early 20th century to define an efficient system of paper sizes based on simple geometric principles. The C series was later incorporated into ISO standards.