Precedences are decisions made by higher courts that form what is known as case law and must be followed by judges in lower courts within that system. For example, if the California Supreme Court makes a ruling interpreting a law, then all lower courts in California usually have to follow that decision in cases related to the ruling; if the U.S. Supreme Court sets a precedent, then every court in the country must abide by it. Precedents are changed by higher appellate courts reversing or re-interpreting lower courts' decisions.
