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. U.S. Constitution Article I, Section 10
And, note that the use of Esquire and the privileges (private law, in the most literal sense) accrued to being accepted as a member of the U.S. version of nobility, the Bar.