Article Four of the United States Constitution: States, Territories, and Federal Guarantees

Major provisions

• Section 1 — Full Faith and Credit: requires states to respect the public acts, records, and judicial proceedings of other states.
• Section 2 — Privileges and Immunities, Extradition: limits state discrimination against citizens of other states, provides for the return of fugitives, and historically included the fugitive slave clause later nullified by the 13th Amendment.
• Section 3 — New States and Territories: gives Congress authority to admit new states and to manage federal territories and property.
• Section 4 — Guarantee and Protection: guarantees a republican form of government for each state and authorizes federal protection against invasion and domestic violence.

In practice the Full Faith and Credit clause is used to enforce judgments and recognize legal acts across state lines — for example, probate, contracts, and court decisions. States may, however, set reasonable rules for procedure and public policy can sometimes limit immediate recognition; Congress also has the power to prescribe how official acts are proven and their effect.

The Privileges and Immunities provision is interpreted to prevent states from discriminating against citizens of other states with respect to fundamental rights, such as access to courts or basic commercial protections, while allowing differential treatment for certain regulatory purposes (for instance, resident tuition at state universities). Extradition between states is a legal process by which a person charged with a crime in one state can be returned from another state.

Section 3's grant to Congress to admit new states has produced the practice that new states enter the Union on an "equal footing" with existing states. The same section gives Congress authority over federal territories and property, shaping how territories evolve toward statehood or different political status.

The Guarantee Clause promises each state a republican form of government and a federal duty to protect states from invasion and, at the request of a state executive, to aid against domestic violence. Courts have often treated some aspects of this clause as nonjusticiable political questions, so its enforcement commonly occurs through political and executive channels rather than litigation.

Notable facts: the fugitive slave provision of Article IV was rendered inoperative by the abolition of slavery; the clause on full faith and credit has been central to interstate enforcement of judgments; and disputes about privileges and immunities reflect ongoing tensions in federalism and state sovereignty.