Federalism and Interstate Commerce
The Constitution regulates the relations between the states.
The full faith and credit clause (Article IV, Section 1) requires that states honor federal Supreme Court judgments and other legal decisions.
Under Article IV, Section 2, states must extend the same rights to residents of other states that are extended to their own residents.
Finally, the extradition clause (Article IV, Section 2), states are required to extradite (hand over) any citizen who flees to their state to avoid prosecution or incarceration in another state.