Incorporates three ISO Standards in the field of preferred numbers, viz. ISO 3, which sets out the preferred numbers; ISO 17, which is a guide to the use of preferred numbers; and ISO 497, which is a guide to the choice of a series of preferred numbers and of series containing more-rounded values of preferred numbers. The Standard defines preferred number and tabulates the basic series R 5, R 10, R 20 and R 40, and the exceptional R 80 series; it includes general guidance on their use, and deals particularly with the more-rounded values tabulating both a first rounding and a second rounding. An appendix discusses precision of the values and regularity of the ratio.