'A' series covers flat sheets, stationery and books, 'B' series is an alternative
intermediate size range, normally used for items such as posters and wall charts and
includes some envelope sizes and the ‘C’ series applies exclusively to envelopes.
Other common sizes have evolved from this system and include under size DL, over size
DL and oversize C5 which are suitable for use with automatic mailing machines where
the contents are automatically inserted. (See also ISO,
ISO Paper Sizes.)