Article IV governs how the states relate to each other and to the Union: each state must respect the others' laws, court judgments, and citizens; new states join on Congress's terms; and the federal government guarantees every state a republican form of government and protection against invasion. It also contained the fugitive slave clause, the Constitution's starkest concession to slavery, nullified by the Thirteenth Amendment.
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.
A court judgment, public record, or law from one state must be respected in every other, a divorce, debt judgment, or adoption finalized in Ohio is valid in Texas. Congress can set the rules for how such records are proved across state lines.
The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States.
A Person charged in any State with Treason, Felony, or other Crime, who shall flee from Justice, and be found in another State, shall on Demand of the executive Authority of the State from which he fled, be delivered up, to be removed to the State having Jurisdiction of the Crime.
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
Visit another state and you carry your basic rights with you, states cannot systematically discriminate against outsiders in fundamental matters. Someone charged with a crime who flees to another state must be extradited back. The third paragraph is the fugitive slave clause: it required states to return escaped enslaved people, written without using the word "slavery." The Thirteenth Amendment abolished slavery and rendered it dead letter; it remains on the page as part of the historical record.
New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Congress decides when territories become states, the power behind every admission from Vermont in 1791 to Hawaii in 1959 (the map's Statehood lens colors each state by that date). No state can be carved out of an existing state without both that state's and Congress's consent. West Virginia's 1863 admission, during the Civil War, is the famous test of this clause. Congress also governs the territories and federal property directly, which is why territories like Puerto Rico and Guam are administered under federal law without full statehood.
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
Every state must have a representative government, no state may become a monarchy or dictatorship, and the federal government must defend states against invasion and, when a state asks, against internal uprisings. Courts treat disputes under the Guarantee Clause as "political questions" for Congress and the President rather than judges, so it is rarely litigated but foundational.
