In the United States, law derived from judicial decisions is also referred to as common law. This type of law operates by application of precedent and so is also known as precedential law. Because case law is not enacted by a legislature, it is also a type of non-statutory law.
In the civil law tradition, case law formally plays a minor role compared to the status of the civil code; however, judicial interpretation of the civil code, interpreting the legal meaning of the code's provisions, clarifying them, and providing for unforeseen developments, is often referred to as a jurisprudence constante. In France, the jurisprudence constante of the Cour de cassation (for civil and penal cases) or the Conseil d'État (for administrative cases) is in practice equivalent to case law, and is considerably important in certain areas such as labor law and administrative law. In particular, the Conseil d'État and the Constitutional Council have adopted "fundamental principles" that statutes and regulations must follow, even when those principles were not explicitly written in statutes.
In the common law tradition, case law interprets laws, via precedents, based on how prior cases have been decided. Case law governs the impact court decisions have on future cases. Unlike most civil law systems, common law systems follow the doctrine of stare decisis in which lower courts usually make decisions consistent with previous decisions of higher courts.
Generally speaking there is no direct oversight that appellate courts have over a court of record. If a lower court judge acts against precedent and the case is not appealed, the lower court decision will stand. This may occur more frequently than has been documented as an appeal is usually quite expensive to prepare. A court may rule against a precedent that is outdated — that is, the court believes that developments or trends in legal reasoning render the precedent inapplicable. In doing so, the court may wish to help the law evolve by ruling against precedent and thereby indirectly inducing a losing party to appeal. If the court successfully induces the appeal, the appellate court will have an opportunity to review the lower court's decision and may adopt the lower court's reasoning and overturn previous case law. This may happen several times as the case works its way through intermediate appellate systems. Lord Denning, first of the High Court of Justice, later of the Court of Appeal, provided a famous example of this evolutionary process in his development of the concept of estoppel starting in the world renowned High Trees case: Central London Property Trust Ltd v. High Trees House Ltd  K.B. 130.
The different roles of case law in civil and common law traditions create differences in the way that courts render decisions. Common law courts generally explain in detail the rationale behind their decisions with numerous citations to previous decisions and other authority (called ratio decidendi). By contrast, decisions in civil law jurisdictions are generally very short, referring only to statutes. The reason for this difference is that these civil law jurisdictions adhere to a tradition that the reader should be able to deduce the logic from the decision. Courts in civil law jurisdictions also render their decisions so that, in some cases, it is somewhat difficult to apply previous decisions to the facts presented in future cases. Some pluralist systems, such as Scots law in Scotland and so-called civil law jurisdictions in Quebec and Louisiana, do not follow these traditions as these systems have been heavily influenced by the Anglo-American common law tradition; however, their substantive law is firmly rooted in the civil law tradition. Because of their position between the two main systems of law, these types of legal systems are sometimes referred to as mixed systems of law.
Law professors in common law traditions play a much smaller role in developing case law than professors in civil law traditions. Because court decisions in civil law traditions are brief and not amenable to establishing precedent, much of the exposition of the law in civil law traditions is done by academics rather than by judges; this is called doctrine and may be published in treatises or in journals such as Recueil Dalloz in France. Historically, common law courts relied little on legal scholarship; thus, at the turn of the twentieth century, it was very rare to see an academic writer quoted in a legal decision (except perhaps for prominent jurists such as Coke and Blackstone). Today academic writers are often cited in legal decisions as persuasive authority; often, they are cited when judges are attempting to implement reasoning that other courts have not yet adopted, or when the judge believes the academic's restatement of the law is more compelling than can be found in precedent. Thus common law systems are adopting one of the approaches long common in civil law jurisdictions.
In federal or multi-jurisdictional law systems there may exist conflicts between the various lower appellate courts. Sometimes these differences may not be resolved and it may be necessary to distinguish how the law is applied in one district, province, division or appellate department. Usually only an appeal accepted by the court of last resort will resolve such differences and, for many reasons, such appeals are often not granted.