The ISO system in general, ISO 216 and 269 provide 3 series (A, B and C, A is the base series, B is the geometric mean between two sizes of the A series, C is the geometric mean between the A and B series at a given index, and is thus mostly used for envelopes for the A series: an envelope Cn will hold an An sheet without folding, and of course a Cn envelope will fit an A(n-1) folded in two)
The ISO system in general, ISO 216 and 269 provide 3 series (A, B and C, A is the base series, B is the geometric mean between two sizes of the A series, C is the geometric mean between the A and B series at a given index, and is thus mostly used for envelopes for the A series: an envelope Cn will hold an An sheet without folding, and of course a Cn envelope will fit an A(n-1) folded in two)