Case law and common law overlap heavily, but they are not the same thing. Common law is the centuries-old legal tradition in which law develops through judicial decisions rather than legislative codes. Case law is the written record of those decisions — the individual opinions judges produce when they resolve disputes. One is the tradition; the other is the material that tradition is made of.
A useful way to picture it: common law is the forest, case law is the individual trees. Each opinion adds to, refines, or occasionally reshapes the larger body of law that guides future decisions.
The Difference in Plain Terms
Common law traces back to medieval England, where royal courts began applying uniform standards instead of relying on inconsistent local customs. Judges started grounding their rulings in earlier decisions, and over time a shared body of law emerged that applied equally across the realm. That tradition traveled to Britain’s colonies and became the foundation of the legal system in the United States and many other countries.
Case law is how that tradition gets built and documented. Every time a court publishes an opinion explaining its reasoning, that opinion becomes part of the case law. Collectively, those opinions form the evolving common law. Legislatures do not create common law directly. It grows one decision at a time, as courts resolve real disputes.
People often use the two phrases interchangeably in everyday conversation, and in most contexts no harm is done. The more precise way to think about it is that case law is the mechanism through which common law develops. If you are reading a single court opinion, you are reading case law. If you are describing the broader tradition of judge-made rules that opinion sits inside, you are describing common law.
What Inside an Opinion Actually Becomes Law
Not every sentence in a judicial opinion carries equal weight, and this matters for understanding how case law feeds common law. The part that counts is the holding — the court’s specific answer to the legal question it needed to resolve. The holding is what binds future courts.
Everything else a judge writes that isn’t essential to resolving the case is dicta, short for the Latin obiter dictum, meaning “something said in passing.” Dicta might include thoughts on hypothetical scenarios, broader policy observations, or commentary on issues the court did not need to decide. Dissenting opinions fall into the same category. Dicta is not binding, though a later judge may find it persuasive when facing a new question.
So when people say “case law,” what they functionally mean — in the sense that shapes future disputes — is the collection of holdings. Those holdings are the pieces that accumulate into common law over time.
How Precedent Carries the Common Law Forward
The engine that keeps the system coherent is stare decisis, a Latin phrase meaning “to stand by things decided.” Under this doctrine, courts generally follow the rules established in earlier cases when the same legal question comes up again. This is what gives common law its predictability: you can look at past rulings and reasonably anticipate how a court will handle a similar situation.
Precedent comes in two forms:
- Binding precedent is a rule set by a higher court that lower courts in the same jurisdiction must follow. If a state supreme court rules that a particular contract clause is unenforceable, every trial court in that state must apply the same rule.
- Persuasive precedent comes from courts in other jurisdictions or from lower courts, and carries no obligation. A judge may look to a persuasive ruling from another state when facing a question no court in their own state has addressed, but they can reach a different conclusion.
Stare decisis is a strong presumption, not an unbreakable rule. Higher courts can overturn their own earlier decisions, though they treat it as exceptional. The U.S. Supreme Court weighs several factors before overruling a past decision, including the quality of the original reasoning, whether the rule has proven workable, whether later decisions have undermined its logic, whether relevant facts have changed, and how heavily people have relied on the existing rule.1Justia. Janus v. AFSCME, 585 U.S. ___ (2018) When the Court does overrule itself, the new decision generally applies retroactively to all cases still open on direct review.2Justia. Harper v. Virginia Dept. of Taxation, 509 U.S. 86 (1993)
The authority of any piece of case law also depends on which court issued it. Article III of the U.S. Constitution places federal judicial power in one Supreme Court and whatever lower courts Congress creates.3Legal Information Institute. U.S. Constitution Article III Supreme Court decisions bind every other court in the country. State supreme courts hold equivalent authority over the interpretation of their own state’s laws. Appellate courts below that level bind the trial courts within their jurisdiction. A trial court’s ruling resolves the dispute in front of it but generally does not create binding precedent for other judges.
Common Law vs. Civil Law Systems
The clearest way to see what makes common law distinctive is to compare it with its main alternative. Most of continental Europe, Latin America, and parts of Asia follow civil law systems, which work quite differently.
In a civil law system, the primary source of law is a comprehensive written code drafted by the legislature and regularly updated. Judges apply the code to the facts of each case, but their individual opinions do not create binding precedent. A civil law judge functions more like an investigator applying a rulebook. A common law judge actively shapes the law through each published opinion. In civil law countries, the code itself — not past rulings — is the authoritative source.
This contrast is where the term “common law” earns its weight. In a common law country, case law isn’t just a reference point; it is law in its own right. In a civil law country, case law exists but doesn’t carry the same force.
Where Case Law Fits Alongside Statutes and Regulations
Common law isn’t the only kind of law operating in the United States, so case law does more than just build the common law tradition. Much of modern case law involves judges interpreting statutes passed by legislatures. Statutes often use broad language that cannot anticipate every scenario, and when disputes arise over what a statute means, courts issue opinions interpreting it. That interpretation becomes part of the case law and guides future application of the statute. Judges start with the ordinary meaning of the words, consider how they fit within the broader structure of the law, and may review legislative history to understand what the lawmakers intended.
One boundary worth noting: not every government ruling counts as case law in the traditional sense. Federal agencies employ administrative law judges who hear disputes involving regulations, from Social Security disability claims to environmental enforcement. These judges operate within the executive branch rather than under Article III. Their rulings can be appealed into the federal courts, and once an Article III court reviews an agency decision and publishes an opinion, that opinion becomes part of the case law.
So the short answer to the question is this. Case law is not identical to common law, but it is the raw material from which common law is made. Every published judicial opinion contributes a holding; those holdings, applied through stare decisis, make up the common law tradition that governs much of American life.