Common sizes of sheets of paper used in the graphic arts and printing industry outside of North America are ISO Sheet sizes. The A4 size is roughly equivalent to letter size used in the US/North America. A4 paper is 210 x 297 millimeters, slightly longer and narrower than the North American letter size of 8.5 x 11 inches. The typical uses for the A4 size are letterhead, newsletters or magazines, forms, catalogs, and use in office machines including copiers and printers.
ISO (International Standards Organization) uses the metric system and sheets are measured in millimeters. The A0 sheet (841x1189mm) is one square meter and smaller sizes are A1, A2, A3, A4.
A4 is one-fourth of an A0 sheet. Two A4 sheets equal an A3. C4, C5, or C6 envelopes are typically used with A4 paper (none, 1, or 2 folds respectively).