In 1959 the British Standards Institute adopted a standardized system of sizes for printing and writing papers. This new standard was based on the International Standards Organisation (ISO) sizes in use in most other countries.

The A B C Series of Sizes
This practical and economical system is based on a rectangle with an area of one square metre, referred to as AO (841 x 1189mm). In addition there are two other series of sizes included in the system - B and C. The B series of sizes are based on a basic sheet size of 1000 x 1414mm and these are used mainly for posters, wall charts and some maps.

The C series of sizes are based on a basic sheet size of 917 x 1297mm and is used for envelopes. These C series envelope sizes are formulated to take A series paper exactly. For example, C4 for A4, C5 for A5 and C6 for A6. One of the most popular envelope sizes, DL, is however an exception to this. The DL designation is derived from the German DIN Lang, ‘DIN’ being the German equivalent of the ISO standard, and ‘Lang’ meaning long.

All ‘A’ series paper sub-divisions denote the actual finished size of the sheet. Printers often prefer to print on paper that is a little larger than these A sizes to allow printing and finishing machines to “grip” the sheet and allow trimming, to clean up the end product without reducing the final trimmed size. These larger sizes are prefixed ‘R’.

Some work needs even larger sheet sizes to allow printed images to “bleed off” the trimmed size or for large “grip” allowances, or to allow quality control images to be printed in the trim margins. These sizes are prefixed ‘SR’.

A number of derivatives of these standard international sizes are becoming increasingly popular for the economical production of particular formats. This can be a problem i.e. 707 x 1000mm, 720 x 1000mm and 720 x 1020mm can all be described as B1. Avoid confusion by specifying dimensions required.