http://electrical-engineering-portal.com/resources/knowledge/theorems-and-laws/thevenins-theorem

Thevenin’s Theorem states that it is possible to simplify any linear circuit, no matter how complex, to an equivalent circuit with just a single voltage source and series resistance connected to a load, or:

Any linear circuit containing several voltages and resistances can be replaced by just a Single Voltage in series with a Single Resistor...