Last month when we talked about the fact that North American business stationery differs in size from business stationery in the rest of the world (our American 8-1/2x11 sheet of paper is 6 millimeters wider and 18 millimeters shorter than the standard A4 paper), we mentioned that the European ISO system for business stationery is based on a height-to-width ratio (the aspect ratio, for those interested in the technical term) of the square root of two. In other words, for every one unit of the shorter dimension (the width) of a piece of paper, its longer dimension (the height) will measure (in units) the square root of two. (来源：英语学习门户 http://www.EnglishCN.com)

The advantages of this mathematically elegant and metric system are manifold: it is considered aesthetically pleasing; the aspect ratio is identical to that of the side of a square to its diagonal; and the ratio is maintained when paper is divided (thus making magnification or reduction relatively simple).

But we neglected to mention one small detail in our earlier program, and it was a detail that gave more than one alert math-lover pause: the square root of two, the basis of this system, is not a rational number; it is expressed as an infinite decimal. It starts off as 1.4142135, and, for the purposes of sizing papers, is rounded to 1.4142.