By connecting two or more resistors in SERIES between the positive and negative supplies, you can split the voltage down into a series of steps. The size of these steps depends on the resistor values used. For instance, if two resistors, one 220 and another 470 ohms, were connected in series in that order between a 9 volt supply and ground, you would find that the voltage at the point in between the resistors is 6.13V. The first 220 ohm resistor loses 2.87V across it. The calculation is based on the ratio of sizes of the resistors used. Variable resistors can the middle pin, assuming they have one, to acquire the divided voltage.