Common law is a legal system, used in the United Kingdom, in which law develops through judicial precedents and accumulated case law rather than one comprehensive written code, supporting rule of law by making judges' past decisions binding guides for future cases.
Common law is the legal tradition where judges' rulings actually create law. When a UK court decides a case, that decision becomes a precedent, and future courts are expected to follow it in similar situations. Over centuries, those stacked-up rulings form a body of law that exists alongside (not instead of) statutes passed by Parliament. Think of it as law built case by case from the bottom up, rather than written all at once in a single code.
In AP Comp Gov, common law matters because it's the UK's answer to a question every course country has to answer: where does law come from, and who interprets it? The UK relies on precedent and an uncodified constitution. Iran bases its legal system on Islamic Sharia law interpreted by clerically trained judges. China practices rule by law, where the judicial system serves the decisions of the Chinese Communist Party. Common law is one of those three models, and the exam loves making you compare them.
Common law lives in Topic 2.8 (Judicial Systems) in Unit 2: Political Institutions, supporting learning objective 2.8.A, which asks you to describe the structure and functions of judiciaries. The essential knowledge here is comparative by design. The UK's common law system supports rule of law, where everyone (including the government) is bound by legal limits. That stands in deliberate contrast to China's rule by law, where the CPC controls judicial appointments and the courts answer to the party, and Iran's judiciary, whose main job is keeping the legal system consistent with Sharia. If you can explain why a UK judge cites precedent while an Iranian judge cites religious law, you've got the heart of Topic 2.8. Common law also feeds into the bigger UK puzzle of parliamentary sovereignty, because UK courts apply precedent but cannot strike down acts of Parliament, which limits judicial power in a way that's unique among course countries.
Keep studying AP® Comparative Government Unit 2
Rule of Law (Unit 2)
Common law is a mechanism; rule of law is the principle it serves. Because precedent binds future judges, common law makes legal outcomes predictable and applies the same rules to everyone, which is exactly what rule of law requires. The UK uses the first to deliver the second.
Parliamentary Sovereignty (Unit 2)
Here's the twist in the UK system. Judges make law through precedent, but Parliament can override any precedent with a statute, and UK courts have no power of judicial review over acts of Parliament. The UK Supreme Court (created in 2009) interprets law without being able to strike down legislation, a built-in tension the exam asks about directly.
Religious Law Basis for Judiciary (Unit 2)
Iran is the cleanest contrast to common law. Iranian judges must be trained in Islamic Sharia law, the head of the judiciary is appointed by the Supreme Leader, and the system's job is keeping law consistent with religious doctrine. UK law evolves through cases; Iranian law is anchored to a religious source.
Appointment and Confirmation Process (Unit 2)
How judges get their jobs shapes how independent a legal system can be. UK Supreme Court justices are chosen through an independent selection commission rather than partisan confirmation, which is part of why the UK's common law courts can credibly enforce precedent against the government, while China's party-controlled appointments produce courts that serve the CPC.
Common law shows up as a UK identifier in multiple-choice questions, usually asking you to pick the feature that distinguishes the UK judiciary from other course countries or to spot the tension between parliamentary sovereignty and judicial independence. Stems also test the 2009 creation of the UK Supreme Court as the development that reshaped the courts' relationship with Parliament. On the free-response side, the 2025 SAQ Q3 asked test-takers to compare limits on judicial power in two course countries, and common law is exactly the kind of concept that anchors that comparison. The move the exam rewards is contrast: UK precedent-based courts under a sovereign Parliament, versus China's party-subservient judiciary, versus Iran's Sharia-based system. Don't just define common law; pair it with a second country.
These get blended together because the UK has both, but they're different categories. Common law is a type of legal system, describing where law comes from (judicial precedent and case law). Rule of law is a principle, describing who law applies to (everyone, including the government). A country could theoretically have a codified legal system and still have strong rule of law. The reverse pairing is China's situation: it has written codes but practices rule by law, where the CPC stands above the legal system rather than under it.
Common law is the UK's legal system in which judicial precedents and case law, rather than a single written code, form the basis of law.
In common law, past court decisions bind future judges in similar cases, which makes legal outcomes predictable and supports rule of law.
The UK's common law courts still operate under parliamentary sovereignty, so the UK Supreme Court interprets law but cannot strike down acts of Parliament.
Common law contrasts with China's rule by law, where courts are subservient to the Chinese Communist Party, and with Iran's judiciary, which exists to enforce Islamic Sharia law.
The creation of the UK Supreme Court in 2009 separated the top court from the House of Lords and is the key 21st-century change in the UK judicial system tested on the exam.
On FRQs, use common law comparatively, pairing the UK with China or Iran to show how legal systems shape the limits on judicial power.
Common law is the legal system used in the United Kingdom where law is built from judicial precedents and case law rather than one comprehensive written code. In Topic 2.8, it's the UK's model for how a judiciary interprets and applies law, supporting rule of law.
No. Because of parliamentary sovereignty, no UK court, including the Supreme Court created in 2009, can invalidate an act of Parliament. UK judges shape law through precedent and statutory interpretation, but Parliament can override any precedent with new legislation.
Common law describes where law comes from (judicial precedent and case law), while rule of law describes who law applies to (everyone, including the government). The UK's common law system is a tool that helps deliver rule of law, but the two terms aren't interchangeable.
The UK's common law courts apply precedent and support rule of law. China practices rule by law, meaning the judiciary serves CPC decisions and the party controls most judicial appointments. Iran's judiciary exists to keep the legal system consistent with Islamic Sharia law, with its head appointed by the Supreme Leader.
Yes. It falls under Topic 2.8 (Judicial Systems) and learning objective 2.8.A. The 2025 SAQ Q3 asked for a comparison of limits on judicial power in two course countries, and common law is a natural anchor for the UK side of that comparison.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.