In electronics, certain types of passive components such as resistors, capacitors and inductors are manufactured with a standardised range of values called preferred values.

For example, in the so-called E12 range, there are twelve values in each decade. These are:

1.0, 1.2, 1.5, 1.8, 2.2, 2.7,
3.3, 3.9, 4.7, 5.6, 6.8, 8.2

This means that E12 resistors exist with values of 3.3 ohms, 33 ohms, 330 ohms, 3300 ohms etc. but not with values such as 2 ohms or 8.3 ohms.

The E12 series is an approximation of a geometric series, with the ratio between adjacent values being or about 1.212. The reason for the choice of twelve values is based on the tolerance of 10% with which some resistors are made. This tolerance means that a resistor marked "27 ohms 10%" could be as low as 24.3 ohms or as high as 29.7 ohms. Therefore there would be no point in selling resistors whose values were closer together than E12, since the tolerance would be greater than the spacing between values.

For 2% and 5% tolerance components, the E24 series is used. This contains the following values:

1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 
1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 
3.3, 3.6, 3.9, 4.2, 4.7, 5.1, 
5.6, 6.2, 6.8, 7.5, 8.2, 9.1

For 1% tolerance components, the E96 series is used, and for 0.5% the E192 series. These series are too long to list here!

For components with very wide tolerances such as large-valued capacitors, the E6 series is used, as follows:

1.0, 1.5, 2.2, 3.3, 4.7, 6.8

The series of preferred values are specified in IEC (International Electrotechnical Commission) publication 60063 (1963-01).

The series of values for the current ratings of fuses does not adhere to the IEC standards, but uses ten equal intervals per decade. After some arbitrary rounding, this results in the series:

1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8

This external page may be used to explore colour codes and E-ranges: http://www.okaphone.nl/calc/resistor.shtml