Mainly, standard paper sizes used in printers are based on the ISO A-series system, with sizes including A0, A1, A2, A3, A4, A5, etc., and smaller formats in between. A4 is the most widely used paper size around the world, for both office, educational, and personal printing. A3 and A2 sheets are often utilized for posters, drawings, large-format printing such as wall charts as well, etc. Small paper sizes like A5 & A6 are typically used for leaflets, handouts, and books. The ratio of each format within the series is such that every next one is only half the size of its preceding one, and the proportion remains constant, which makes it so straightforward to enlarge or reduce printing as needed between various paper formats.