Article IV of the U.S. Constitution deals with state citizenship, the relationship between states, and the relationship between the states and the federal government. It requires states to give "full faith and credit" to decisions made by other state courts. The Framers of the Constitution kept this idea from the Constitution's predecessor, the Articles of Confederation.
More on each of the clauses in Article IV below.
Full Faith and Credit
In Mills v. Duryee, an 1813 case out of New York, the Supreme Court interpreted Article IV to mean that one state cannot reopen a court case that another state has resolved. However, in a later case, the court created the "public policy" exception to the full faith and credit clause - finding that the Constitution does not force one state to substitute another state's law for its own. In other words, where there is a conflict of laws, a state can choose to prioritize its own laws over those of a different state.
The full faith and credit clause is an essential piece of constitutional law, but it is most often applied in the family law context. For example, the Violence Against Women Act invokes the full faith and credit clause to require states to honor orders for protection and child custody orders issued in different states. Before gay marriage was made legal nationwide, legal scholars grappled with the question of whether the full faith and credit clause required states that banned same-sex marriages to recognize marriages carried out in other states. In the mid-1990s, the Defense of Marriage Act attempted to legislate around this issue but was ultimately struck down under the Constitution's equal protection clause rather than the full faith and credit clause.
Privileges and Immunities
The privileges and immunities clause of the Constitution requires states to treat residents and nonresidents the same by giving them the same "privileges and immunities. In other words, state law cannot favor in-state residents over citizens of other states. The phrase "privileges and immunities" appears again in the Fourteenth Amendment, which guaranteed due process and equal protection under the laws for all citizens after the Civil War.
The guarantee clause of Article IV requires that states employ a "republican form of government," meaning they must use the electoral process. An individual state cannot decide to be a monarchy, even if the populace votes to do so. However, the Supreme Court has interpreted the guarantee clause to mean that states can choose what type of election process to use. As James Madison wrote in the Federalist Papers, "the States may choose to substitute other republican forms."
Article IV also addresses how Congress can admit new states into the union.
Learn More About Article IV
What Does Article IV of the Constitution Say?
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.
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.
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.
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.