A court decision is a judicial determination of parties’ rights and obligations reached by a judge based on facts and law. Decisions include the text of a case, legal arguments, and court orders. They are often cited by other judges to support or challenge a specific point of law. A court’s decisions form case law, a collection of precedents (rules of the courts established from previous judgments). Precedents are binding on lower courts only if there is no higher authority such as legislative statute or another court ruling that overrules them.
When the Supreme Court decides a case, the Justices who participate in the case write a written opinion which sets out their views of how the case should be decided. This is usually known as the majority opinion. A Justice who disagrees with the outcome of the case but agrees with the reasoning used by the majority may write a concurring or dissenting opinion. These explain alternative grounds for reaching a different result to the majority. Dissenting opinions can sometimes be more influential than the majority opinion, as was the case with Justice John Marshall Harlan’s dissent in Plessy v Ferguson, where he condemned the “separate but equal” principle behind racial segregation.
Although not all cases are important, a decision is considered important when it changes or clarifies existing law and establishes new principles of law. A judge’s choice to cite or reference a particular past decision reflects his or her understanding that the decision is important and has a high normative value.