The preferred numbering system has played a major role in the development of metric standards. This is a geometrical series of numbers adopted worldwide. Its first known application was in the 1870's by Charles Renard, a French army captain who reduced the different diameters of rope for military balloons from 425 to 17. The R5, R10 and R20 series refers to the Renard 5 (first-choice sizes 60 % increments), Renard 10 (second-choice sizes 25 % increments) and Renard 20 (third-choice sizes 12 % increments) series of preferred numbers standardized in ISO 3. Preferred sizes are shown in ANSI B4.2 and ISO 497.
You might want to extend the preferred metric size range, which, is shown from 4 to 40, simply by multiplying or dividing the numbers shown in Table 4-1 by 10 or multiples of 10.
The preferred metric sizes are used for nominal sizes in countries where the metric system has been in use for several years. Here is how the preferred metric nominal sizes were developed and how these chosen sizes reflect preferred metric standard sizes for threaded fasteners, steel plates, sheets, bars, etc already in use throughout the world.