The E12 series is so called because 12 'Preferred Values' of resistor, each having a tolerance range of +/-10% covers all values from 10Ω to 100Ω. This range of values is called a decade, and the next higher range (decade) in the E12 series covers values between 90Ω (100Ω -10%) and 902Ω (820Ω +10%) and so on.
