Common law is judge-made law. Common law is a feature of most countries previously colonized by Great Britain, where it originated. In continental Europe, an alternative system called civil law developed, where judges do not have the power to create law through interpretation. In civil-law jurisdictions, only the legislature may create law. A jurisdiction is an area where power may be exercised.

In a common-law system, when an appellate court hears cases and writes opinions, rules of law are created, formed, and shaped. After a particular legal issue has been decided in a jurisdiction, there is a high probability that subsequent cases that present the same legal issue will use the same rule of law generated from already-decided cases regarding the same legal issue. This policy is known as stare decisis, or “let the decision stand.” This is how a precedent is formed, though precedents may shift or change over time. Precedents also may be entirely overturned, though that is rare. Precedents and stare decisis allow us to anticipate the behavior of others and to gauge the legality of our own actions.

Legal reasoning is used by attorneys to argue for a particular outcome in a case and by judges when rendering decisions. At its most basic form, legal reasoning involves first identifying the legal question, which is the issue in dispute. Then, the rule of law that applies to that issue is identified. The rule of law may be drawn from precedent, for example. The facts of the case are analyzed against the rule of law to reach a supportable conclusion. This method of legal reasoning is referred to as the IRAC method, which is an acronym for issue, rule, analysis, and conclusion.

Common law is an important source of law in those many areas that are reserved to the states to regulate. A state may exercise its police powers to regulate the safety, health, and welfare of its citizens, for example. The laws implemented in these areas may give rise to laws in divergent areas, such as property law (e.g., zoning regulations), so-called vice laws (e.g., restrictions on vice business activities in certain areas or during certain days), and domestic relations (e.g., laws relating to marriage and adoption). It’s also important to note that precedents vary among different jurisdictions because precedents created by one jurisdiction are not binding in other jurisdictions.

Most administrative agencies are created by the legislature. At the federal level they are created by Congress, and at the state level they are created through the state legislative bodies. Administrative agencies may be thought of as a delegation of congressional authority to area experts in particular fields, so that those experts can engage in limited lawmaking, adjudicative procedures, and investigations within their particular purviews. Laws made by administrative agencies are called rules or regulations. Administrative agencies are created by enabling legislation, which sets forth the agencies’ jurisdictional boundaries, rule-making procedures, and other information relating to agencies’ scopes of power.