times a power of ten.

It is called the 'E12' series because there are 12 values, spaced exponentially across a change in value of a power of ten.

When an engineer designs a circuit, they work out the component values which make it work as well as possible. This may lead to values like 149 Ohms or 255,334 Ohms. The manufacturer then usually chooses the closest value from the E12 series. Instead of 149 and 255,334 Ohms they would use 150 and 270k. Since each E12 value is about 20% bigger than the last, the nearest E12 is never in error by more than about 10%. In practice this kind of error in resistor values rarely affects the behaviour of a circuit. The large quantity of E12s used means that their price is low, keeping manufacturers happy!

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