Common Law

Common law represents a legal structure based on precedent and tradition. It comes to us mainly from the British system, dating back to the medieval world, when custom dictated much of the legal relationships between people (like, say, who gets a witch's property when she fails the drowning test...or whether witnessing a comet is sufficient cause for annulling a marriage).

Because we were a British colony, there are aspects of common law in our legal system. When a judge quotes the precedent of a previous judge's ruling to explain the decision she made, she's using common law principles.

The other kind of legal decision-making is called statutory law. In statutory law, some law-making body puts a rule on the books, and judges and lawyers just look at what the law says. Obviously, that's a large part of the U.S. legal system.

However, laws can be confusingly written, or strange situations can come up where it isn't clear how the law should get applied. In these cases, lawyers and judges look to precedent (common law principles) to craft their arguments and make their decisions.