Article I, Section 10, Clause 1:
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.
Within the sense of the Constitution, bills of credit signify a paper medium of exchange, intended to circulate between individuals, and between the government and individuals, for the ordinary purposes of society. It is immaterial whether the quality of legal tender is imparted to such paper. Interest-bearing certificates, in denominations not exceeding ten dollars, that were issued by loan offices established by the state of Missouri and made receivable in payment of taxes or other moneys due to the state, and in payment of the fees and salaries of state officers, were held to be bills of credit whose issuance was banned by this section. 1 The states are not forbidden, however, to issue coupons receivable for taxes, 2 nor to execute instruments binding themselves to pay money at a future day for services rendered or money borrowed. 3 Bills issued by state banks are not bills of credit; 4 it is immaterial that the state is the sole stockholder of the bank, 5 that the officers of the bank were elected by the state legislature, 6 or that the capital of the bank was raised by the sale of state bonds. 7