C10

The C10 paper size has dimensions of 28 x 40 mm, making it the smallest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.001 m2, C10 is rarely used as an envelope size. As the last size in the C series based on geometric means between A and B series sheets, C10 falls between A10 (26 x 37 mm) and B10 (31 x 44 mm). While not a typical envelope format, C10 could potentially be used for mailing very small, thin items requiring slightly more space than a small jewelry or coin envelope. However, C10 sees little practical use. The C10 and other C sizes originate from early 20th century German efforts to define paper sizes based on geometric principles, later incorporated into ISO standards.