---
title: "Common Law Systems — AP Gov Definition & Exam Guide"
description: "Common law systems make court precedents a primary source of law. Learn how stare decisis ties into judicial legitimacy for AP Gov Topic 2.9 and Unit 2."
canonical: "https://fiveable.me/ap-gov/key-terms/common-law-systems"
type: "key-term"
subject: "AP US Government"
unit: "Unit 2"
---

# Common Law Systems — AP Gov Definition & Exam Guide

## Definition

A common law system is a legal system where court decisions and precedents serve as a primary source of law, not just written statutes. In AP Gov, it's the foundation for stare decisis, the doctrine that courts follow past rulings in similar cases (Topic 2.9, Legitimacy of the Judicial Branch).

## What It Is

A common law system is one where judges' past rulings actually become law. When a court decides a case, that decision (the [precedent](/ap-gov/key-terms/precedent "fv-autolink")) binds future courts facing similar facts. The United States inherited this system from England, and it's why Supreme Court opinions matter so much. A ruling isn't just a one-time answer to one dispute; it sets the rule everyone follows afterward.

The engine that makes common law run is **stare decisis**, Latin for "let the decision stand." Under stare decisis, courts follow legal precedents when deciding cases with similar facts. This gives the law two things at once: stability (people can predict how courts will rule) and adaptability (courts can refine or, occasionally, overturn precedents as the country changes). That balance is exactly what [AP Gov](/ap-gov "fv-autolink") [Topic 2.9](/ap-gov/unit-2/legitimacy-judicial-branch/study-guide/VJ8DnmbCug0vKC25idPk "fv-autolink") is about. Precedent is a huge source of the judicial branch's legitimacy, because it makes the Court look like it's applying consistent legal principles rather than just voting its politics.

## Why It Matters

This term lives in **[Unit 2](/ap-gov/unit-2 "fv-autolink"): Interactions Among Branches of Government**, specifically **Topic 2.9, Legitimacy of the Judicial Branch**, and it directly supports learning objective **AP Gov 2.9.A**: explain the role of [legal precedent](/ap-gov/key-terms/legal-precedent "fv-autolink") in judicial decision making. Here's the core tension the CED wants you to see. In a common law system, precedent gives the unelected judiciary its legitimacy, but precedents aren't permanent. Ideological changes in the Court's composition (driven by presidential appointments) have led the Court to establish new precedents or reject old ones. So the common law system explains both why the Court is respected and why fights over Supreme Court nominations are so intense. Whoever shapes the Court shapes which precedents survive.

## Connections

### Precedent and Stare Decisis (Unit 2)

Precedent is the building block of a common law system. [Stare decisis](/ap-gov/key-terms/stare-decisis "fv-autolink") is the rule that courts use those blocks. You can't explain one without the other, and AP Gov 2.9.A asks you to explain exactly how precedent drives judicial decision making.

### [Judicial Review (Unit 2)](/ap-gov/key-terms/judicial-review)

[Judicial review](/ap-gov/key-terms/judicial-review "fv-autolink") (established in Marbury v. Madison) gives courts the power to strike down laws. Common law is what makes those strikes stick, because each ruling becomes binding precedent. Marbury itself is a precedent that has held for over two centuries.

### [Brown v. Board of Education (Units 2-3)](/ap-gov/key-terms/brown-v-board-of-education)

Brown (1954) is the classic example of a common law system adapting. The Court rejected the [Plessy v. Ferguson](/ap-gov/key-terms/plessy-v-ferguson "fv-autolink") precedent of "separate but equal" and replaced it with a new one. Overturning precedent is rare, which is exactly why Brown is such a big deal.

### [Checks and Balances (Unit 2)](/ap-gov/key-terms/checks-and-balances)

Because precedents come from judges, the other branches check the common law system indirectly. Presidents appoint justices who may reject existing precedents, and Congress can rewrite statutes in response to rulings. The New Deal conflict, where FDR clashed with a Court that kept striking down his programs, is the CED's illustrative example.

## On the AP Exam

Multiple-choice questions usually test this term through comparison or application. A classic stem asks for the key difference between common law and civil law systems regarding precedent (answer: common law treats precedent as binding law; civil law relies on written legal codes). You should also be ready to connect common law to stare decisis and to explain why a Court with new appointees might overturn precedent. No released FRQ has used "common law systems" verbatim, but the concept shows up constantly in the SCOTUS Comparison FRQ, where you have to explain how a required case (like Brown v. Board) relates to a non-required case. That whole question type only exists because we have a common law system where precedents connect cases across decades.

## Common law systems vs Civil law systems

In a common law system (like the U.S.), judicial precedents are a primary source of law, and courts must follow earlier rulings under stare decisis. In a civil law system (like France), comprehensive written legal codes are the primary source of law, and judges apply the code rather than building law from prior decisions. The quick test: if past court rulings bind future courts, it's common law; if judges mainly consult a written code, it's civil law.

## Key Takeaways

- In a common law system, court decisions and precedents are a primary source of law, not just written statutes.
- Stare decisis is the doctrine that courts follow legal precedents when deciding cases with similar facts, and it's the engine of any common law system.
- Precedent gives the unelected judicial branch legitimacy by making rulings look consistent and principled rather than political.
- Precedents can change. Ideological shifts on the Supreme Court caused by presidential appointments have led the Court to establish new precedents or reject old ones, like Brown v. Board overturning Plessy.
- The key contrast on the exam is with civil law systems, which rely on written legal codes instead of binding judicial precedent.
- This concept lives in Topic 2.9 and supports learning objective AP Gov 2.9.A on the role of precedent in judicial decision making.

## FAQs

### What is a common law system in AP Gov?

It's a legal system where court decisions and precedents are a primary source of law, so judges' past rulings bind future courts in similar cases. In AP Gov, it appears in Topic 2.9 as the foundation of stare decisis and judicial legitimacy.

### What's the difference between common law and civil law systems?

Common law systems treat judicial precedent as binding law, while civil law systems rely on comprehensive written legal codes. The U.S. is a common law country, which is why Supreme Court precedents like Marbury v. Madison still control cases today.

### Does a common law system mean precedent can never be overturned?

No. Stare decisis means courts usually follow precedent, but the Supreme Court can and does reject precedents, especially after ideological shifts caused by new presidential appointments. Brown v. Board of Education (1954) overturning Plessy v. Ferguson is the textbook example.

### How is common law different from statutory law?

Statutory law is written law passed by a legislature like Congress, while common law is judge-made law built from court precedents. In the U.S. both exist together, and courts use precedent to interpret what statutes mean.

### Why does the common law system make the judicial branch legitimate?

Because following precedent makes the Court's rulings predictable and principled, people accept decisions even from unelected judges. That's the central idea of Topic 2.9 and learning objective AP Gov 2.9.A.

## Related Study Guides

- [2.9 Legitimacy of the Judicial Branch](/ap-gov/unit-2/legitimacy-judicial-branch/study-guide/VJ8DnmbCug0vKC25idPk)

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