A Series  

A0  841×1189 
A1  594×841 
A2  420×594 
A3  297×420 
A4  210×297 
A5  148×210 
A6  105×148 
A7  74×105 
A8  52×74 
A9  37×52 
A10  26×37 
B Series  
B0  1000×1414 
B1  707×1000 
B2  500×707 
B3  353×500 
B4  250×353 
B5  176×250 
B6  125×176 
B7  88×125 
B8  62×88 
B9  44×62 
B10  31×44 
C Series  
C0  917×1297 
C1  648×917 
C2  458×648 
C3  324×458 
C4  229×324 
C5  162×229 
C6  114×162 
C7  81×114 
DL  110×220 
C7/6  81×162 
The standard was originally adopted by DIN in Germany in 1922, although some of the formats contained therein were independently invented in France during its revolution and later forgotten.
ISO 216 defines three series of paper sizes: A, B and C. Series C is primarily used for envelopes.
Paper in the A series format has a 1:√2 aspect ratio, although this is rounded to the nearest millimetre. A0 is defined so that it has an area of 1 m², prior to the above mentioned rounding. Successive paper sizes in the series A1, A2, A3, etc., are defined by halving the preceding paper size parallel to its shorter side, again prior to rounding. The most frequently used paper size is A4, which is 210 × 297 mm².
The B series formats are geometric means between the A series format with the same number and the A series format with one higher number. For example, B1 is a geometric mean between A1 and A0. Likewise, the C series formats are geometric means between the B series format with the same number and the B series format with one higher number, e.g. C2 is the geometric mean between B2 and B1. (There is also an incompatible Japanese B series, defined on the basis of the arithmetic mean instead of the geometric mean.)
The C series formats are used mainly for envelopes. An A4 page will fit into a C4 envelope. If you fold the A4 page so that is A5 in size, it will fit in a C5 envelope and so on.
Prior to the adoption of ISO 216 worldwide, many different paper formats were used internationally. These formats did not fit into a coherent system and were defined in terms of nonmetric units. The ISO 216 formats are organized around the ratio 1:√2; two sheets next to each other together have the same ratio, sideways. This simplifies copying two A4 sheets in reduced size on one, and copying an A4 sheet in magnified size on an A3 sheet or copying half an A4 sheet in magnified size on an A4 sheet. The main holdbacks in this area are the United States of America and Canada, which continue to use the Letter, Legal, and Executive system (Canada uses a Pseries of sizes, which are nothing more than the US paper sizes rounded to metric dimensions).
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