Common law is a legal system in which the law is built mainly from judges’ written decisions rather than from a single comprehensive code passed by a legislature. When a court resolves a dispute, the reasoning in that decision becomes part of the legal record, and later judges facing similar facts are expected to follow it. Roughly 38 countries use some form of this system, including the United States, the United Kingdom, Canada, Australia, and India.
How a Common Law System Works
There is no master rulebook. The rules accumulate, decision by decision, as courts resolve real disputes. Over decades and centuries, this layering produces a detailed body of law covering contracts, injuries, property, employment, and much more.
The process is adversarial. Two opposing sides present their arguments to a judge, and often a jury decides the facts. The judge applies the relevant legal principles and, in doing so, may refine or extend them for the next case. That is why common law is sometimes called judge-made law: judges don’t just apply rules handed down from a legislature, they actively develop the rules themselves through their written opinions.
Precedent and Stare Decisis
The engine that makes the whole thing work is precedent. Under a doctrine called stare decisis, courts follow the legal principles set out in earlier decisions on similar issues. A trial court is bound by the appellate courts above it, and appellate courts follow their respective supreme courts. The hierarchy gives the system its consistency. If a state supreme court ruled ten years ago that a certain type of contract clause is unenforceable, lower courts in that state will reach the same conclusion in similar cases today.
Precedent isn’t a straitjacket. A judge can “distinguish” a case by explaining why its facts differ meaningfully from an earlier ruling, making the old rule inapplicable. Higher courts can also overrule their own prior decisions outright, though the bar is high.
When Courts Break From Precedent
The U.S. Supreme Court has said overturning a prior decision requires “special justification” or at least “strong grounds” beyond simple disagreement with the earlier reasoning.1Congress.gov. Constitution Annotated – ArtIII.S1.7.2.2 Stare Decisis Doctrine Generally Courts weigh things like the quality of the original reasoning, whether the rule has proven workable in practice, whether later decisions have eroded it, whether circumstances have changed, and whether people and businesses have structured their affairs around the old rule. The Court laid these factors out explicitly in Janus v. American Federation of State, County, and Municipal Employees in 2018.2Justia. Janus v. AFSCME, 585 U.S. (2018)
The reliance factor often carries the most practical weight. Even when a court believes an old decision was wrong, it may leave it in place rather than upend settled expectations across an industry. As Justice Brandeis put it, in most matters “it is more important that the applicable rule of law be settled than that it be settled right.”1Congress.gov. Constitution Annotated – ArtIII.S1.7.2.2 Stare Decisis Doctrine Generally
How Common Law Fits With Statutes
Common law and statutory law are not competing systems. They work together. Statutes are written by legislatures (Congress, state legislatures, city councils), while common law develops through court decisions. When a legislature passes a statute on a topic previously governed only by common law, the statute takes priority. Courts then interpret and apply that statute, and those interpretations themselves become a new layer of common law.
Sometimes legislatures codify common law principles, essentially writing judge-made rules into formal statutes without changing their substance. The Uniform Commercial Code, adopted in some form by every state, did exactly this for large portions of commercial and contract law. Other times, legislatures deliberately override common law rules by passing a statute that changes the standard courts had been applying. And where statutes are silent or ambiguous, common law fills the gaps.
The practical result is that most legal disputes involve both. A personal injury case might run on a common law negligence standard but hit a statutory cap on damages. A contract dispute might turn on common law offer and acceptance while also involving a consumer protection statute.
Common Law vs. Civil Law
Most of the world’s legal systems fall into one of two broad categories: common law or civil law. The difference is fundamental. In a common law system, judge-made precedent is a primary source of law, and courts have broad authority to develop legal principles through their decisions. In a civil law system, the law is organized into comprehensive written codes, and judges apply those codes to the facts without the same power to create binding precedent.
Civil law systems descend from Roman law and are found across continental Europe, Latin America, and much of Asia and Africa. Scholars and legislators do most of the heavy lifting there, drafting and updating the codes that judges apply. A civil law judge’s role is closer to an investigator who establishes the facts and matches them to the relevant code provision. In a common law courtroom, the judge acts more as a referee while opposing lawyers present their cases, and the judge’s written opinion can shape the law going forward.
Louisiana is the only U.S. state that operates under a civil law tradition for much of its private law, a legacy of its French and Spanish colonial history.3U.S. District Court Eastern District of Louisiana. An Introduction to Louisiana’s Civil Law Tradition Its code traces its lineage back to Roman law rather than English common law. In practice, Louisiana’s system has absorbed plenty of common law influence over the years, particularly in commercial law, but the structural distinction is real.
Where Common Law Shows Up in Everyday Law
Common law has left its fingerprints on nearly every area of American law. A few stand out for how heavily they still rely on judge-made principles.
Tort Law
Tort law, the body of law governing civil wrongs like negligence and trespass, is largely a common law creation. When someone sues after a car accident, a slip-and-fall, or a defective product injury, the framework they are using was built by courts. A negligence claim requires showing that the defendant owed a duty of care, breached that duty, and caused actual harm. Those elements were refined through centuries of court decisions. Legislatures have layered statutes on top (damage caps, statutes of limitations, comparative fault rules), but the core framework remains common law.
Contract Law
The basic rules of contract formation — offer, acceptance, consideration, and mutual assent — grew out of common law. Courts worked out these principles case by case, deciding when a promise was legally binding and when it wasn’t. While statutes like the Uniform Commercial Code now govern many commercial transactions, common law still controls contracts for services, real estate, and other areas outside the UCC’s scope. When a court interprets an ambiguous contract term or decides whether a party materially breached an agreement, it is typically applying common law principles.
Employment Law
The default rule in American employment, that either side can end the relationship at any time for almost any reason, is a common law doctrine. At-will employment wasn’t passed by any legislature. It emerged from court decisions in the late nineteenth century and became the dominant standard across the country. Courts have also carved out the exceptions. Most states recognize that an employer can’t fire someone for reasons that violate public policy, such as retaliating against an employee who filed a workers’ compensation claim. Many states recognize implied contract exceptions as well, where an employer’s handbook or consistent practices create a reasonable expectation that termination will happen only for cause.
Common Law Marriage
The phrase “common law” also appears in the context of marriage, and the term causes constant confusion. A common law marriage is a court-recognized union formed without a license or ceremony, but it is not created by simply living together for a certain number of years. That is a persistent myth. Most states do not allow new common law marriages at all. Those that still do include Colorado, Iowa, Kansas, Montana, New Hampshire (for inheritance purposes only), South Carolina, Texas, and the District of Columbia, with Oklahoma and Rhode Island recognizing them through case law.4National Conference of State Legislatures. Common Law Marriage by State Where it is recognized, a valid common law marriage carries the same legal weight as a ceremonial one, including for federal tax purposes.5IRS. Revenue Ruling 2013-17 Ending one requires a formal divorce.