"Horizontal" Federalism
The obligations of the states to each other
Full Faith and Credit
Article IV, Section 1  "Full faith and credit shall be given in each state to the public acts, records and judicial proceedings of every other state."

Privileges and Immunities
Article IV, Section 2  "The Citizens of each state shall be entitled to all privileges and immunities of Citizens in the several states."

Article IV, Section 2  "A person charged in any state with treason, felony, or other crime, who shall flee 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."