The legal basis for Common law systems?

In Common Law systems judges base their decisions largely on past decisions of other courts. This has the advantage of movement toward a consistent application of law across geography and time. The system evolved in England and is in place in most US states.

Another approach is sometimes referred to as Napoleonic Code, a system developed in France which is in place in the state of Louisiana. Judges there make decisions based on interpretation of statute law (laws passed by the legislature) only.