In 1952 the IEC (International Electrotechnical Commission) decided to define the resistance and tolerance values into a norm, to ease the mass manufacturing of resistors. These are referred to as preferred values or E-series, and they are published in standard IEC 60063:1963. These standard values are also valid for other components like capacitors, inductors and Zener diodes. The preferred values for resistors were established in 1952, but the concept of the geometric series was already introduced by army engineer Renard in the 1870s. Microhm Electronics also follow these standards and rules for resistors.
 
The standardization of resistor values serves several important purposes. When manufacturers produce resistors with different resistance values, these end up approximately equally spaced on a logarithmic scale. This helps the supplier to limit the number of different values that have to be produced or kept in stock. By using standard values, resistors of different manufacturers are compatible for the same design, which is favorable for the electrical engineer.
 Aside from the preferred values, many other standards related to resistors exist. An example is standard sizes for resistors, or the marking of resistors with color codes or numerical codes. Power ratings of resistors are not defined in a norm, therefore often is deviated from the above described series.
 
As basis the E12 has been developed. E12 means that every decade (0.1-1, 1-10, 10-100 etc) is divided in 12 steps. The size of every step is equal to: Formula for preferred values for resistors 10^(1/12)=1.21
 
One could also say every value is 21% or 1.21 times higher than the last, rounded to whole numbers. Because of this, all resistors with a tolerance of 10% overlap. The series looks as follows: 1– 1.2 – 1.5 – 1.8 – 2.2 – 2.7 – 3.3 – 3.9 – 4.7 – 5.6 – 6.8 – 8.2 – 10 etc. All these values can be powers of ten (1.2– 12 – 120 etc).
 
Next to the E12 series, other series are existing. It is a good practise to specify resistors from a low series when tolerance requirements are not high. The most common series are:
 
E6   20%
E12 10%
E24 5% (also available with 1%)
E48 2%
E96 1%
E192 0.5% (also used for resistors with 0.25% and 0.1%).
 
Microhm Electronics' NLR series follow E6, E12 and E24 standards according to different resistance range. LMK series follow E24 and E96 standards.