For the full text of the U.S. Constitution, see Full Text of the United States Constitution.
Article IV of the United States Constitution addresses relations between the states of the United States.
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.
In brief, states must honor laws and court orders from other states, even if their own state laws differ. For example, a marriage or adoption that is legal in one state must be recognized as legal in all 50 states.
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.
Text abrogated by the 13th Amendment
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.
This section prevents states from discreminating against residents of another state. Section 2 also orders states to return any fugitives from justice to the state from which they are fleeing. Section 2 also gave slave owners the right to reclaim runaway slaves across state lines.
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.
This grants Congress the power to admit new states into the union, but prevents individual states from creating new states inside their own borders, and also establishes the Equal footing doctrine.
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 can-not be convened) against domestic Violence.
The exact meaning of this clause is debated amongst legal scholars and is open to interpretation. Credited to James Madison, this clause is usually understood to mean that each state must be ran as a representative democracy (as opposed to a dictatorship or monarchy) and that the United States has an obligation to defend any state's government that is danger of being overthrown by violent means.