Hello Friends, After Explained A and B series paper size in my previous videos ( Episode Links : B – Series Paper Size Explained : https://youtu.be/kPWNyyYMDE0 and A-series Paper Size Explained : https://youtu.be/7q592aYWcmc ) In this video we will know about C Series Paper Sizes. The C series is usually used for envelopes and is defined in ISO 269. The area of C series sheets is the geometric mean of the areas of the A and 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 and C4 paper fits 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 is 114 mm × 229 mm where the common side to C5 and C6 is 162 mm. The α variables are the distinct first terms in the three geometric progressions of the same common-ratio equal to the square root of two. Each of the three geometric progressions (corresponding to the three series A, B, [[#ISO C|C]) is formed by all possible paper dimensions (length and width) of the series arranged in a decreasing order. This interesting arrangement of dimensions is also very useful – not only it forms a geometric progression with easy to remember formulae, it also has that each consecutive pair of values (like a sliding window of size 2) will automatically correspond to the dimensions of a standard paper format in the series. The tolerances specified in the standard are ±1.5 mm (0.06 in) for dimensions up to 150 mm (5.9 in), ±2 mm (0.08 in) for lengths in the range 150 to 600 mm (5.9 to 23.6 in) and ±3 mm (0.12 in) for any dimension above 600 mm (23.6 in).. We will also know about C0 Paper size, C1 paper size, C2 paper size, C3 Paper Size, C4 Paper Size, C5 Paper Size, C6 Paper Size, C7 Paper Size and C8 Paper Size. Also know about different types of Envelope size and use of different types of in different types of Envelope.