Common law is a body of legal rules built from judicial decisions rather than from statutes written by a legislature. Instead of starting with a comprehensive code that tries to answer every possible question in advance, a common law system grows one case at a time: judges resolve real disputes, their reasoning is written down, and those written opinions become the rules that guide later courts facing similar problems. Roughly 40 countries operate under a purely common law framework today, and another 60 or so blend it with civil, religious, or customary law.
Where the System Came From
Common law originated in medieval England after the Norman Conquest of 1066. Before then, English law was a patchwork of tribal and regional customs that varied from one area to the next. William the Conqueror sent royal judges on circuits through the countryside to settle disputes under a single standardized set of rules, and their decisions were recorded, shared among courts, and treated as authoritative for later cases. Over generations, this accumulation of rulings became “the common law” — common not because it was ordinary, but because it applied commonly across the entire realm.
British colonization carried the system around the world. The United States, Canada, Australia, India, Singapore, and dozens of other countries inherited versions of English common law and adapted them locally. The shared ancestry is why lawyers in Sydney and lawyers in New York often work from similar foundational principles, even though their statutes and recent case law have diverged.
Common Law Compared to Civil Law
The main alternative is the civil law tradition, which dominates continental Europe, Latin America, and much of Asia and Africa. The core difference is where the law lives. In a civil law system, legislatures draft comprehensive codes meant to address every area of legal life — property, contracts, family relations, criminal conduct — and the judge’s job is mainly to apply the right code provision to the facts. Past decisions carry less weight, because the code itself is supposed to supply the answer.
In a common law system, judicial decisions are themselves a source of law. When no statute covers a situation, the judge looks at how earlier courts handled similar problems and reasons by analogy. That makes judges far more powerful as lawmakers in common law countries than in civil law ones, where the legislature holds a tighter monopoly on creating rules. Neither tradition is pure in practice. Common law countries have mountains of statutes, and civil law judges do look to prior decisions for guidance. But the default starting point — code versus case law — shapes how lawyers argue and how law students are trained.
How Precedent Works
The engine that makes common law function is stare decisis, a Latin phrase meaning “to stand by things decided.” Once a court resolves a legal question, other courts facing the same question should reach the same answer. This gives people the ability to predict outcomes, plan their affairs, and trust that the system will not produce wildly different results depending on which judge happens to hear the case.
Not all precedent carries the same weight. A ruling from a higher court in the same jurisdiction is binding: lower courts must follow it whether they agree with the reasoning or not. A decision by a federal circuit court of appeals, for example, controls every district court within that circuit. A ruling from a court at the same level, or from a different jurisdiction entirely, is merely persuasive. A judge can consider it and find its logic compelling, but there is no obligation to adopt it.
This hierarchy is what gives common law its structure. Attorneys researching a client’s problem start by looking for binding precedent from higher courts in their jurisdiction. If none exists on point, they turn to persuasive authority from other courts that have addressed similar facts. The whole system depends on meticulous recordkeeping. Without published opinions, there would be no precedent to follow.
When Courts Overrule Precedent
Stare decisis is a strong presumption, not an absolute rule. Courts can and do overturn their own prior decisions, but the bar is deliberately high. The Supreme Court has identified several factors it weighs before abandoning established precedent: the quality of the original reasoning, whether the rule has proven workable in practice, its consistency with related doctrines, developments since the decision was issued, and how heavily people have relied on the old rule in ordering their lives.1Supreme Court of the United States. Janus v. State, County, and Municipal Employees
Simply thinking an earlier court got the answer wrong is not enough. The justification for overruling has to amount to more than disagreement with the earlier reasoning.2Supreme Court of the United States. Dobbs v. Jackson Women’s Health Organization If courts freely discarded precedent whenever the composition of a bench changed, the predictability that makes stare decisis valuable would disappear.
Courts also have subtler tools short of outright overruling. A judge can distinguish an earlier case by pointing to factual differences that make the old rule inapplicable. Or a court can narrow a prior holding, limiting it to its specific facts so it no longer controls a broader category of disputes. These techniques let the law evolve incrementally without the disruption of reversing a settled rule.
How Judges Make New Law
When a case raises a legal question that no statute and no prior decision squarely addresses, the judge has to reason from existing principles to reach a new conclusion. This is where common law actually grows. A court facing a digital privacy dispute might draw on decades of physical trespass rulings to decide whether accessing someone’s data without consent counts as a legal wrong. The analogy is imperfect, but common law has always evolved by extending old principles to new circumstances.
That adaptability is one of the system’s genuine strengths. Legislatures move slowly. Courts respond in real time to the disputes people actually bring. The tradeoff is unpredictability: two courts in different jurisdictions might reach opposite conclusions on the same emerging issue, and the conflict can persist until a higher court resolves it or a legislature steps in.
When Statutes Override Common Law
Statutes passed by a legislature take priority over judge-made rules whenever they address the same subject. If Congress or a state legislature passes a law that contradicts an existing common law principle, the statute wins. Elected representatives have the authority to update, replace, or overrule judicial precedent through legislation. Statutory law sits above common law.
The relationship runs both ways. When a statute uses a legal term without defining it, courts typically apply whatever meaning the term carried at common law. When statutory language is ambiguous, judges interpret it against the backdrop of existing common law principles. There is even a longstanding presumption that a statute should not be read to change the common law unless the legislature’s intent to do so is clear. Statutes displace old rules while courts fill in the gaps that statutes inevitably leave.
Common Law in the United States
The United States inherited English common law through a deliberate legal act. During the founding era, states passed reception statutes that formally adopted English common law as it existed at the time of independence, so long as it did not conflict with the new Constitution or state laws. Every state except Louisiana built its legal system on that foundation. Louisiana’s legal tradition traces back to French and Spanish colonial rule, and its private law still operates under a civil code rather than accumulated case law.3U.S. District Court Eastern District of Louisiana. Civil Law in Louisiana
Common law still fills enormous gaps in American law today. When you sue a neighbor for negligence, fight a breach of contract claim, or dispute a property boundary, you are often litigating under rules that no legislature wrote. Statutes have expanded to cover more and more ground, but common law remains the default framework for most private disputes.
No Federal General Common Law
One structural rule is worth knowing: federal courts cannot invent their own body of general common law. In 1938, the Supreme Court held in Erie Railroad Co. v. Tompkins that when a federal court hears a case based on diversity jurisdiction — meaning the parties are from different states — it must apply the substantive law of the relevant state, not an independent federal version.4Justia U.S. Supreme Court Center. Erie Railroad Co. v. Tompkins, 304 U.S. 64 (1938) The Court put it bluntly: “There is no federal general common law.”
Federal common law does still exist in a few narrow areas where the federal government’s sovereign interests demand uniform rules, including admiralty and maritime law, disputes between states, and matters touching foreign relations. Outside those pockets, a federal judge sitting in Ohio applies Ohio common law.
Common Law Does Not Create Crimes
In civil disputes, common law can supply the rule when no statute exists. Criminal law works differently. Since 1812, the Supreme Court has held that federal courts have no power to prosecute anyone for a common law crime. The legislature must first define the offense, attach a punishment, and designate which court has jurisdiction.5Justia U.S. Supreme Court Center. United States v. Hudson, 11 U.S. 32 (1812) You cannot be convicted of a federal crime that Congress never wrote into law. Most states follow the same principle. If no statute makes particular conduct illegal, a prosecutor generally cannot charge you with it, no matter how harmful the behavior might seem.
Damages, Injunctions, and the Law-Equity Divide
Common law originally offered only one type of relief: money damages. If someone breached a contract or injured your property, you could sue for compensation, but the court could not order the wrongdoer to actually do anything, such as stop trespassing, hand over a unique item they promised to sell, or tear down an encroaching fence. To get those orders, you had to petition a separate court of equity, overseen by the Crown’s chancellor, which could grant injunctions, require specific performance of a contract, or impose other non-monetary remedies.
In 1938, the Federal Rules of Civil Procedure merged the procedural sides, eliminating separate law and equity dockets.6Federal Judicial Center. Federal Rules of Civil Procedure Merge Equity and Common Law From that point forward, plaintiffs filed a single civil action and could request both damages and equitable relief in the same case. The old distinction still matters, though. Whether you are entitled to a jury trial depends on whether your claim is legal or equitable in nature, and before a court will issue an injunction, you still need to show that money alone will not make you whole.
Common Law Doctrines You Have Probably Heard Of
The ideas above can feel abstract until you see them working in specific areas. The following doctrines are all pure common law in origin, even where statutes have since modified them.
Common Law Marriage
Common law marriage lets two people be legally married without a license or ceremony. The traditional requirements are a mutual agreement to be married, cohabitation, and publicly holding yourselves out as spouses, such as using the same last name, filing joint tax returns, or introducing one another as husband and wife.
Far fewer states allow this than most people assume. Only about eight jurisdictions currently permit new common law marriages to be formed: Colorado, Iowa, Kansas, Montana, New Hampshire, South Carolina, Texas, and Utah, along with Rhode Island and Oklahoma through case law rather than statute.7National Conference of State Legislatures. Common Law Marriage by State Several other states, including Alabama and Georgia, once recognized common law marriage but have cut off new formations after a specific date. A common law marriage validly created in a state that allows it is generally recognized by other states, even those that do not permit their own residents to form one.
Negligence
Negligence may be the most heavily litigated common law doctrine in existence. To win, a plaintiff generally must prove four things: the defendant owed a duty of care, breached that duty, caused the plaintiff’s injury, and that actual damages followed. The standard for breach is the “reasonable person,” a hypothetical individual who exercises ordinary prudence under the circumstances. It is an objective test: what matters is what a prudent person in your position would have done, not what you personally thought was safe. The entire structure, duty through damages, was built by judges, not legislators.
Contract Formation
The basic rules for creating an enforceable contract were developed almost entirely through common law. A valid contract requires an offer, acceptance, consideration (something of value exchanged by each side), and the legal capacity of both parties to enter the agreement. These elements emerged through centuries of judicial decisions about which promises the legal system would enforce. The Uniform Commercial Code now governs the sale of goods, but for service contracts, employment agreements, and most other non-goods transactions, the baseline rules still come from judge-made law.
Adverse Possession
Adverse possession allows someone who openly occupies another person’s land for a long enough period to eventually claim legal ownership. The doctrine sounds strange at first, but it clears up stale title disputes, rewards productive use of land, and penalizes owners who neglect their property for decades. The common law requirements are that the possession be actual, open and visible, hostile (meaning without the owner’s permission), exclusive, and continuous for a statutory period that varies widely by state, from a few years to twenty or thirty. The doctrine originated in common law and has since been modified by statute in every state, a good illustration of how legislative and judicial law interact over time.