C1

The C1 paper size has dimensions of 648 x 917 mm, making it the second largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.56 m2, C1 is an oversized envelope format. As part of the C series based on geometric means between A and B series sheets, C1 falls between A1 (594 x 841 mm) and B1 (707 x 1000 mm). While not common, C1 envelopes can be useful for mailing large documents, posters, artwork, and other oversized items that require an envelope bigger than standard business or personal sizes. The C series traces back to early 20th century efforts in Germany to define paper sizes based on simple geometric principles, later incorporated into ISO standards.