Editing Paper size (section)

===C series<span class="anchor" id="ISO C"></span><span class="anchor" id="DIN C"></span><span class="anchor" id="SIS C"></span>===
[[File:C size illustration2.svg|thumb|right|A size chart illustrating the ISO C series]]
The C series is defined in [[ISO 269]], which was withdrawn in 2009 without a replacement, but is still specified in several national standards. It is primarily used for envelopes. The area of C series sheets is the geometric mean of the areas of the [[#ISO A|A]] and [[#ISO B|B series]] sheets of the same number; for instance, the area of a C4 sheet is the geometric mean of the areas of an A4 sheet and a B4 sheet. This means that C4 is slightly larger than A4, and slightly smaller than B4. The practical usage of this is that a letter written on A4 paper fits inside a C4 envelope, and both A4 paper and C4 envelopes fit inside a B4 envelope.

Some envelope formats with mixed sides from adjacent sizes (and thus an approximate aspect ratio of 2:1) are also defined in national adaptations of the ISO standard, e.g. DIN C6/C5 (also known as C65) is 114&nbsp;mm × 229&nbsp;mm where the common side to C5 and C6 is 162&nbsp;mm. This format allows an envelope holding an A-sized paper folded in three, e.g. for the C65, an A4.