Paper sizes

The Majority of popular paper sizes are known as 'A', 'B' or 'C' sizes. The 'A' Series being the most common. A4 is the most common paper size in the world, known widely as 'Letterhead' size. A5 covers half the area of A4, A6 is half the area of A5 and so on. Conversely, A3 is double the area of A4, and so on up to A0, the largest in the series at 841 x 1189mm.
Additionally to these, we also use several other sizes:

DL (210 x 99mm): DL is a third of an A4 and although usable as a booklet or brochure, is most commonly used for Compliment Slips.

Square 148mm and Square 210mm: Both popular Square sizes used for booklets and brochures.