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ISO 269 describes the C series paper. C series is typically used to define envelope sizes and is closely related to both A and B series paper. Since both A and B paper are widely used, it makes sense to size envelopes for that paper. For example, folding an A4 sized piece of paper in half will fit sngly into a C5 sized envelope.

Format Width x Height (mm) Width x Height (inches)
C0 917 x 1297 mm 36.1 x 51.1 inches
C1 648 x 917 mm 25.5 x 36.1 inches
C2 458 x 648 mm 18 x 25.5 inches
C3 324 x 458 mm 12.8 x 18 inches
C4 229 x 324 mm 9 x 12.8 inches
C5 162 x 229 mm 6.4 x 9 inches
C6 114 x 162 mm 4.5 x 6.4 inches
C7 81 x 114 mm 3.2 x 4.5 inches
C8 57 x 81 mm 2.2 x 3.2 inches
C9 40 x 57 mm 1.6 x 2.2 inches
C10 28 x 40 mm 1.1 x 1.6 inches