E12 is about being +/- 5% of your target resistance using only a singular resistor. A couple of notes: 1. E12 kits plays well with +/-5% tolerance. You're cov

E12 is about being +/- 5% of your target resistance using only a singular resistor. A couple of notes: 1. E12 kits plays well with +/-5% tolerance. You're covering the entire spectrum of resistances possible with your kit, within the error-specs of your resistor. 2. Trimmer potentiometers (cheap pots designed for only ~200 cycles of use or less) are the key component needed for most analog / accurate circuits. ------- You can usually design a circuit with +/- 5% accuracy (nominal), and then have one or two key trimmer-pots tune to 0.1% accuracy at the end. Ex: You might have one trimmer-pot control an a voltage offset (ex: from an instrumentation amplifier), and a second pot control the scale / multiplier (ex: op-amp multiplier and/or resistor divider). You may have 20 to 30 resistors all across your analog circuit, all within +/- 5% tolerances for various reasons. But these two control points at the very end can "trim" away your error and reach 0.1%, or better, accuracy, in practice. But you have to be "close enough", and have "linear enough" behavior. E12 (aka: resistors all within 10% of each other) seems to be enough for most such projects. --------- Digital circuits on the other hand, don't really care too much about this stuff. So you end up with a bunch of 100nF capacitors with 1k resistors just all over the project. That's fine too. You should build a stockpile of the components you plan to use. E12 resistor kits + a set of trimmer pots are for analog circuits. At least if you want 0.1% or 0.01% tolerances (3 or 4 digits).