In a common law system, judges are obliged to make their rulings as consistent as reasonably possible with previous judicial decisions on the same subject. The Constitution accepted most of the English common law as the starting point for American law. Situations still arise that involve rules laid down in cases decided more than 200 years ago. Each case decided by a common law court becomes a precedent, or guideline, for subsequent decisions involving similar disputes. These decisions are not binding on the legislature, which can pass laws to overrule unpopular court decisions. Unless these laws are determined to be unconstitutional by the Supreme Court, they preempt the common law precedent cases. Judges deciding cases are bound by the new law, rather than the precedent cases.

To better understand how the common law works, assume that there is a hypothetical drug, Zoneout, that is a psychoactive drug with some medical uses but a high potential for abuse: It is addictive and users lose their interest in going to work.

If Congress writes the statute regulating Zoneout very clearly and specifically—a complete ban on prescribing or using Zoneout—then the court’s role is limited; if the physician prescribes Zoneout, then the physician has violated the statute and is guilty of a crime. All the cases involving prescriptions for Zoneout will look the same and the law will not evolve. But assume the statute is vague: no prescriptions for dangerous drugs. Then the court will have to decide under which circumstances Zoneout is a dangerous drug and when it is permissible to use it.

Assume that the court decides that Zoneout is a dangerous drug for treating workplace stress. That decision is then published and made available to the public. When the next case of a prescription for Zoneout comes before the court, the judge would be expected to follow the previous decision (the precedent) or to explain why it did not apply. The next case involves a Zoneout prescription for a patient with severe anxiety secondary to cancer treatment. The judge rules that Zoneout is not a dangerous drug under these facts because the risk of addiction is outweighed by the benefits of suppressing the anxiety. As more judicial opinions are written on prescribing Zoneout, it will become clearer when it is legal to use it and when it is prohibited. These opinions are the common law precedent on the prescription of Zoneout. They tell a physician when it is permissible to use Zoneout.

The value of a common law system is that the law can be adapted to situations that were not contemplated by the legislature. There are two disadvantages. First, judges must follow the precedent cases. If they do not, then it is impossible to predict what the law is. The second is that with hundreds of cases being decided every day, it is hard to keep up with the relevant decision. It is not unusual for several courts to be deciding cases on the same subject at the same time, with no good way to coordinate their opinions. Frequently the courts will reach different conclusions about the law. The state court in San Francisco might ban the use of Zoneout in the workplace, but the court in Los Angeles might allow it. Until the California Supreme Court resolves the issue, medical care providers in the two different regions are facing different laws. This type of split also happens between federal courts of appeal, sometimes with three or four parts of the country under different interpretations of a given federal law.

The alternative to the common law system is called a civil law system. In a civil law country, the legislatures pass very specific statutes, and these are applied by the courts. Each judge who decides a case looks to the statute, rather than the previous cases, for guidance. In theory, in ambiguous cases each judge is free to reinterpret the statute as necessary to fit the facts of the specific case. Although this interpretation need not draw on previous decisions by other judges, civil law judges do try to ensure some consistency in the application of the law by taking into consideration previous court decisions. Louisiana retains some of the civil law procedures that were in force before it joined the United States.