"Total federal debt can increase in two ways. First, debt increases when the government sells debt to the public to finance budget deficits and acquire the financial resources needed to meet its obligations. This increases 'debt held by the public'. Second, debt increases when the federal government issues debt to certain government accounts, such as the Social Security, Medicare, and Transportation trust funds, in exchange for their reported surpluses. This increases 'debt held by government accounts'. The sum of 'debt held by the public' and 'debt held by government accounts' is the total federal debt. Surpluses reduce debt held by the public, while deficits raise it. […] Congress has always placed restrictions on federal debt. The form of debt restrictions, structured as amendments to the Second Liberty Bond Act of 1917, evolved into a general debt limit in 1939. Congress has voted to raise the debt limit 11 times since 2001, due to persistent deficits and additions to federal trust funds. Congress raised the limit in June 2002, and by December 2002 the U.S. Treasury asked Congress for another increase, which passed in May 2003. In June 2004, the U.S. Treasury asked for another debt limit increase and again in October 2004, which was enacted on November 19, 2004. In 2005, reconciliation instructions in the FY2006 budget resolution (H.Con.Res. 95) included a debt limit increase. After warnings from the U.S. Treasury, Congress passed an increase that the President signed on March 20. In 2007, Congress approved legislation (H.J.Res. 43) to raise the debt limit by $850 billion to $9,815 billion that the President signed September 29, 2007."
CRS Report for Congress, RL31967