A more thorough explanation:

Common law is a type of law that comes from court decisions instead of written laws. In the United States, common law was originally based on English common law, but now it can be created by American courts. Common law can be used to make decisions in cases where there is no specific law that applies.

For example, in the case of Gamble v. United States, the Supreme Court discussed the role of common law in the legal system. The court explained that common law can be used to make decisions based on previous court cases, or by using similar cases as a guide.

Common law is usually created at the state level, but there is also a limited amount of federal common law. Federal common law is created by federal courts when there is no specific federal law that applies to a case.

For example, in the case of Rodriguez v. FDIC, the Supreme Court struck down a federal common law rule about corporate tax refunds. The court explained that federal common law can only be used when it is necessary to protect federal interests.

When a common law rule is created by a court, the state legislature can choose to turn it into a written law. This is called codifying the law. The legislature might do this to make the law more permanent, or to change it in some way.

For example, in the case of Dynamex Operations West, Inc. v. Superior Court, the California Supreme Court created a test for determining whether workers were employees or independent contractors. The California legislature then turned this test into a written law called California Labor Code Section 2750.3.

These examples show how common law can be used to make decisions in cases where there is no specific law that applies. They also show how common law can be turned into written laws by state legislatures.