C4 - What is C4?

Definition of C4:

An international standard for paper size, most commonly used for envelopes. The C4 size is defined as measuring 229mm wide by 324mm high (with a tolerance of +/- 2mm). A sheet of A4 paper will fit into a C4 envelope.

Here is the Harvard-style citation to use if you would like to reference this definition of the word C4:
Label Planet (2020) What is C4? | C4 Definition. Available at: https://www.labelplanet.co.uk/glossary/c4/ (Accessed: January 1, 2023).