When one is buying, say, an electric lamp for use in the home, the normal range of lamps available is 15, 25, 40, 60, 100 W and so on. These watt values approximately follow a geometric progression, roughly giving a uniform percentage change in light emission between consecutive sizes. In general, the relationship between the sizes of a commodity is not random but based on a system of preferred numbers.

Preferred numbers are based on R numbers devised by Colonel Charles Renard. The principal series used are R5, R10, R20, R40 and R80, and subsets of these series. The values within a series are approximate geometric progressions based on common ratios of ⁵√10, ¹⁰√10, ²⁰√10, ⁴⁰√10 and ⁸⁰√10, representing changes between various sizes within a series of 58% for the R5 series, 26% for the R10, 12% for the R20, 6% for the R40 and 3% for the R80 series.

Further details on the values and use of preferred numbers may be found in BS 2045:1965. The rounded values for the R5 series are given as 1.00, 1.60, 2.50, 4.00, 6.30 and 10.00; these values indicate that the electric lamp sizes given above are based on the R5 series. Many of the standards in use are based on series of preferred numbers and these include such standards as sheet and wire gauges, nut and bolt sizes, standard currents (A) and rotating speeds of machine tool spindles.