As easy as A B C…
The A series of paper sizes was adopted in Europe in the 19th century, and is currently used all around the world apart from the USA and Canada. The largest of these is A0, which is 841mm x 1189mm. Although strange looking numbers you will find if you multiply these dimensions the sheet surface area is 999.949mm or very nearly one square meter.
One of the quirks of this size is that when the sheet is halved, the aspect ratio remains the same. So all the sizes from A0 to A8 are all proportionally the same. This system means enlarging and reducing of images/pages can occur without any loss of image or blank margins, and that when sheets are folded they will make a booklet of the next A size down.
Because of the way paper is made and brought this sizing is important, you will waste less paper if you are printing to a standard size. There is also something familiar about an A4 brochure or an A5 leaflet though you will find other sizes do stand out as something different.