Common Law Explained

Common law relies on judicial decisions. If you’re in a common law legal system, like America, you need to read both statutes and court decisions to understand the law.

What is a common law system?

A common law system relies on both statutes and legal precedents established in court decisions.

Common law systems vary by place and time. For example, in old England, the king only set very high-level laws, and judges were left to fill in most of the rest.

In the United States today, much of the statutory law is based on old common law. The legal system still uses a common law approach to interpret statutes and fill in any blanks.

While common law is often referred to as the unwritten laws, this isn’t really a good way of thinking about it. Common law is typically written down in case law.

How does case law work?

Case law is the written decisions of judges.

For example, if a judge rules that someone who runs a red light is liable for a resulting accident, that ruling becomes part of the case law.

Common Law Courts

In most legal systems, there are multiple levels of judges. For example, United States federal law falls under the jurisdiction of three levels of federal judges

There are U.S. district judges covering each state or parts of larger states. Their decisions can be appealed to a Court of Appeals covering groups of states. Finally, the United States Supreme Court has the ultimate say.

Many states follow a similar system but may have different names for their courts. For example, in New York, the Supreme Court is the trial court, and the Court of Appeals is the highest court. The Appellate Division sits in the middle.


Case law is also known as precedent. Judges typically follow past decisions. Otherwise, there would be no common law.

A common law judge may depart from past decisions if they feel that the current case is different from those decisions. Their judicial opinion would explain what’s different in this case. If the new decision stands on appeal, future judges would follow the newly expanded decision-making process in similar cases.

In rare cases, judges may also decide that past precedent was simply wrongly decided. They may say that a statute was interpreted incorrectly or the previous court exceeded its authority. Lower-level judges who don’t follow precedent are likely to have their decision overturned on appeal.

The precedent covering a jurisdiction is usually considered to be the same court or higher. For example, a Kansas federal district judge is expected to follow previous decisions by other Kansas district judges, the 10th Circuit Court of Appeals (which covers Kansas), and the U.S. Supreme Court. That judge could consider decisions from other states or appeals circuits but isn’t bound by them.

Significant disputes over the common law, such as varying interpretations of federal statutes in different districts or circuits, are resolved by appealing to a higher court.

Common Law vs. Civil Law

The main difference between civil and common law is that a civil law system doesn’t have laws outside of the legal statutes.

A civil law judge is expected to resolve a case purely by applying the legislative statutes. Judicial precedent has little to no importance to future cases.

If judges in a civil law system are reaching different outcomes in similar cases, it’s considered an issue that needs to be addressed with new civil legislation.

Is common law or civil law higher?

Common law and civil law are two different law systems. Many countries have a legal system derived from common law, while others use law derived from civil law.

In practice, most places have blended the concepts from civil and common law. The legal system has to define which laws have priority.

In the United States, judges must follow both the statutes and Consitution. If a judge makes a decision interpreting a statute, the legislature can overturn that decision by passing a new statute.

If a judge makes a decision based on constitutional law, that decision can only be overturned by amending the Constitution. This isn’t because common law is higher than civil law but because the Consitution is higher than a statute.

Historic Common Law

Much of American law is based on old English common law and even Roman law before that. The old common law is only used to fill in the blanks to the extent that it hasn’t been replaced by statutes or decisions by U.S. judges.

For example, under traditional common law, the distinction between assault and battery is that assault is a threat and battery is physical touching. In many states, the crime of assault is defined as a physical attack, while a threat is a lower degree of assault or classified as something else.

Some differences go beyond just what something is called. For example, under common law, first-degree murder is a premeditated murder. Under the current law of many states, first-degree murder only includes murders under special circumstances such as during another crime, against a police officer, or if multiple people were killed.

Most high school, college, and even law school courses teach traditional common law instead of current federal or state law. If you have a legal issue, it’s important to understand that the law you learned in school may not be the actual law in practice.

Further, if you do your own research and read statutes, it can be hard to know if there is case law that applies or how to interpret it. That’s why it’s usually to get help from a lawyer when you have a legal problem.

What is a common law marriage?

Marriage was originally a common law and religious concept. Today, most marriages are via a marriage license authorized by specific statutes.

The idea of a common law marriage is that two people who live as if they were married for some time may be considered married even if they were never formally married. This might include a number of years of living together, sharing assets, and taking other actions that would make it appear as if they were married to people who weren’t aware of their legal marital status.

Even a common law marriage has to be authorized by statute in most places. The reason is that if a state passes a law on how to get married, you have to follow that law to be married. In addition to the formal marriage license process, some states have a law recognizing and defining common law marriages.

How does common law apply to contract law?

The idea of a contract comes from common law. Because the basics of agreeing to a contract are largely unchanged over time, most existing statutes and precedential court rulings that apply to contracts are based on common law.

However, you should be aware that many places have passed specific laws governing contracts. For example, a tenant in a residential lease often can’t waive certain tenants’ rights via contract even if they sign that they’re waiving those rights. The law overrules that part of the contract.

How does common law apply to criminal law?

An important concept in criminal law is that people should be fully aware of the law. In most places, crimes have to be defined by statute.

The old common law crimes such as assault, battery, murder, and burglary have been codified into current statutes.

Constitutional rights, such as the protection against unreasonable searches and seizures, are more likely to be defined by binding precedent rather than statutes. So the concept of common law is still important in criminal cases.

How do legal professionals know the common law?

With thousands if not millions of court cases per year, it may seem impossible to know the common law.

First, it’s important to understand that most cases in lower courts are very similar. There isn’t a lot of new law created.

Second, court decisions are public record. Legal research tools like Westlaw and LexisNexis make it easy to search for related cases.

Finally, judicial authorities learn the common laws through experience. Over time, they learn what types of cases go strictly by statutes and what cases have a lot of existing common law. That’s why many lawyers choose to work only in a specific area of the law.

If you have an important legal issue and try to understand the established rules on your own, there’s a high chance that you could miss something. It’s usually best to contact a lawyer with experience in that specific practice area.