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Common law and case law have essentially the same meaning in many legal systems, including that of the United States. The body of common law is made up of various case law from different court systems throughout the country. This case law is legally enforceable unless a higher court overrules it or the legislature creates a law that supersedes it.
In the United States, the law is comprised of the Constitution, statutes, and case law. The Constitution, both at the federal and state levels, is considered the "supreme law of the land." It created the separation of powers and vested certain authority in the legislative branch and certain authority in the courts.
The federal and state government are each permitted to make laws on the areas that the Constitution delegated appropriate to them. These laws are called bills or statutes and are published in code books. As long as these government-made laws do not violate Constitutional rights or parameters, the laws are valid.
Statutes and the Constitution, however, are all subject to interpretation as to their meaning. Furthermore, these types of law cannot possibly address every potential legal situation or question that arises. This is where common law — or case law — comes into play.
Case law is law made by judges that interprets or refines statutes and constitutions. Case law, in other words, applies general laws to specific cases, thereby refining the definition of the laws in the process. If no specific statute or Constitutional rule is on point, judges must also determine which related laws and rules they believe the law to be based upon.
Once a judge makes case law, or interprets an existing law, that case law and interpretation is binding on all courts at the same level or lower within the jurisdiction. The case law is binding under the doctrine of stare decisis, which is derived from a Latin phrase that means "stand by and adhere to decisions and not disturb what is settled."
Lawyers and individuals can turn to case law to determine how a law will apply, and can use that case law to govern their behavior. All courts within the jurisdiction thus must apply that case law. Only a court at the same level, or a higher court, can overrule existing precedent or case law.
Under the common law system, only case law within the jurisdiction is binding. This means that a California judge does not have to listen to what a New York judge says about the law. A California judge must, however, listen to what other California judges say about the law, unless they are a higher court and can overrule the existing common law rule.
A good example of how common law can be interpreted differently is common law couples versus actual marriages. I once lived in a state that allowed unmarried couples to have property rights similar to married couples. The neighboring state would not recognize those rights.