C9

The C9 paper size has dimensions of 40 x 57 mm, making it the tenth largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.002 m2, C9 is a rarely used envelope size. As part of the C series based on geometric means between A and B series sheets, C9 falls between A9 (37 x 52 mm) and B9 (44 x 62 mm). While not common, C9 envelopes can potentially be used for mailing very small, slim items that require more space than a standard business or note envelope. However, the slightly larger C8 size sees more use. The origins of C9 and other C sizes date back to early 20th century German efforts to define paper sizes based on geometric principles, later included in ISO standards.