When Congress borrows money on the credit of the United States, bonds are thus legislated into existence and deposited as credit entries in Federal Reserve banks. United States bonds, bills and notes constitute money as affirmed by the Supreme Court (Legal Tender Cases, 110 US 421), and this money when deposited with the Fed becomes collateral from whence the Treasury may write checks against the credit thus created in its account (12 USC 391). For example: suppose Congress appropriates an expenditure of $1 billion.
To finance the appropriation, Congress creates the $1 billion worth of bonds out of thin air [actually, created upon a presumption â see David's comment below] and deposits it with the privately owned Federal Reserve System. Upon receiving the bonds, the Fed credits $1 billion to the Treasury's checking account, holding the deposited bonds as collateral. When the United States deposits its bonds with the Federal Reserve System, private credit is extended to the Treasury by the Fed.
Under its power to borrow money, Congress is authorized by the Constitution to contract debt, and whenever something is borrowed it must be returned. When Congress spends the contracted private credit, each use of credit is debt which must be returned to the lender or Fed.
Since Congress authorizes the expenditure of this private credit, the United States incurs the primary obligation to return the borrowed credit, creating a National Debt which results when credit is not returned. However, if anyone else accepts this private credit and uses it to purchase goods and services, the user voluntarily incurs the obligation requiring him to make a return of income whereby a portion of the income is collected by the IRS and delivered to the Federal Reserve banksters.
http://www.21silver.com/?show=merrill&read=public_money_vs_private_credit