If you look in a components catalogue you will discover that the resistor values
appear to have been chosen by a musician! The 'notes' on the resistor scale 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 and 10.0
times a power of ten.
These values are 'equally spaced' in the same way that notes on a (correctly tuned!) piano
are equally tempered. You can calculated each subsequent value, approximately, by multiplying the last value by:
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!