C3

The C3 paper size has dimensions of 324 x 458 mm, making it the fourth largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.14 m2, C3 is an oversized envelope format. As part of the C series based on geometric means between A and B series sheets, C3 falls between A3 (297 x 420 mm) and B3 (353 x 500 mm). While uncommon, C3 envelopes can be useful for mailing large documents, brochures, and other content requiring more space than a standard business or personal envelope. The C3 and other C sizes originate from early 20th century efforts in Germany to define paper sizes based on geometric principles, later incorporated into ISO standards.