The B series formats are geometric means between the A series format with the same number and the A series format with one lower number. As paper is usually specified in g/m≤, this simplifies calculation of the mass of a document if the format and number of pages are known. Therefore, the area of the pages has been defined to have round metric values. The square-root-of-two ratio does not permit both the height and width of the pages to be nicely rounded metric lengths. The ISO paper sizes are based on the metric system. If you put two such pages next to each other, or equivalently cut one parallel to its shorter side into two equal pieces, then the resulting page will have again the same width/height ratio. This aspect ratio is especially convenient for a paper size. In other words, the width and the height of a page relate to each other like the side and the diagonal of a square. In the ISO paper size system, the height-to-width ratio of all pages is the square root of two (1.4142 : 1).
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