Design for print // Paper Sizes research

The "A" Series - From the book 'The All New Print Production Handbook' - David Bann

This is a system for sizing pawper and was first used in Germany in 1922 where it is still called "Din A". Each size is derived by halving the size immediately above it. Each size is the same as another geometrically as they are halved using the same diagonal. A0 is the first size and is one square meter in area.
A series sizes always refer to the trimmed sheet. The untrimmed sizes are referred to as "RA" or "SRA". Many countries have officially adopted the A series system, the USA being the main exception. The "B" series is used when a size in between any two adjacent A sizes is needed. As quoting just the A or B number can cause confusion, both the A or B size and the size in millimeters should be given when specifying paper 

"B" Series

The B series paper sizes were created in order to provide paper sizes that weren't covered by the A series, but also use an aspect ratio of 1:root2. B sizes are defined as size B(n) being the geometric mean of size A(n) and size A(n-1). The Geometric Means of 2 numbers being the square root of the product of the two numbers.

 RA and SRA - Are untrimmed A series sizes.

International C Series Envelopes - Used to ensure when posting that the A series paper sizes would fit inside a standard envelope size.

There are a few countries like  North America, Canada and Japan that use their own pper sizing systems so they in turn have their own envelope sizing system.