C series paper sizes and dimensions are defined by the ISO 269 international paper size standard. The C series is usually used for envelopes. The area of C series paper is the geometric mean of the areas of the A and B series paper of the same number. For example, 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 bigger than A4, and somewhat smaller than B4. The practical usage of this is that a letter written on A4 paper fits inside a C4 envelope, and both A4 and C4 paper fits inside a B4 envelope.