A sizes (A0-A6)
These are the UKs standard paper sizes. The smaller sizes (A6, up to A4) either fit or fold-to-fit standard C-sizes of envelope. To give you an idea of scale, A6 is postcard-sized and A4 is used for letterheads. Confusingly though, the standard US letter size is different from the UKs - useful to know if producing stationery for an American company!