Article I. LEGISLATIVE DEPARTMENTThere is more interesting stuff in this section which restricts the power of the states.
§ 10. Powers Denied to the States
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Control of the Congress.
No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
The states may not impair the obligations of contract. What that means is that states cannot impair the obligations of legal contracts in existence at the time of the legislation. Of course states have made all sorts of encroachments in this area and there is much case law discussing what is and what is not an impairment of contracts.
The prohibition on states entering into compacts with other states is interesting. There are all sorts of interstate compacts in existence today, such as an interstate driver's license compact in the recognition of another state's suspension of privileges and the sharing of information. There is a Multistate Tax Compact. Certainly, Congress has given consent for some Compacts, but it would be odd if the states could agree between themselves on nothing at all without the consent of Congress.
In Virginia v. Tennessee, 148 U.S. 503, 519, 13 S.Ct. 728, 734, 37 L.Ed. 537 (1893), the Supreme Court decided that the application of the Compact Clause is limited to agreements that are "directed to the formation of any combination tending to the increase of political power in the States, which may encroach upon or interfere with the just supremacy of the United States." The case involves the boundary between Virginia and Tennessee that in 1802 had been "marked with great care by the commissioners of the States, with five chops on the trees in the form of a diamond, at such intervals between them as they deemed sufficient to identify and trace the line." The Court said that the states could agree between themselves on the "definition of a true and ancient boundary."