The C series is commonly used for envelopes and its dimensions are defined by the ISO 269 International Standard for paper sizes. The size of a C series envelope falls between its A and B counterparts and is usually used to accompany the A and B series papers. So if you’re looking for envelopes to fit an A or B series paper, the C series might be a good place to start.
As a general rule of thumb, the A series paper sizes fit into a C series envelope of the same number. This means that if you need an envelope for your A4 letter, it’s best to go for a C4 envelope. By taking the same A4 letter and folding it in half, it becomes an A5. Here, the most appropriate size to use will be the C5 envelope, which will be the same size as a C4 envelope folded in half.
Similar to the A and B series, the aspect ratio of the C series is 1:√2. Thus, each sheet of paper can be folded once or twice to neatly fit into a smaller envelope.
The most common envelopes for business use are the C4, C5, C6 and DL sizes.