 The voltage divider is one of the fundamental circuits used in electronics. You'll find it used again and again in schematics once you know to look for it. In its most common form, the resistive divider, two resistors are connected in series between a voltage source and ground to create a reference voltage. The relationship between these resistor values determines that reference voltage available at the node between them. We can use this simple equation to determine that voltage. So, with a 5V supply voltage, a first resistor value of 10k ohms and a second value of 20k ohms, the voltage available between the resistors should be about 3.33 volts. And so it is. Thanks again, math.