(1) Treasury debt held by the public: $11.3 trillion, 73 percent of GDP for fiscal 2012. This is the most common measure of the national debt. Reflecting past annual deficits, it represents what must be borrowed through sales of Treasury bills, notes and bonds. In 2007, the figures were only $5 trillion and 36 percent of GDP. Today’s levels — as a share of GDP — are the highest since World War II’s immediate aftermath.

(2) Gross federal debt: $16 trillion for 2012, 103 percent of GDP. This definition includes the “debt held by the public” (above) plus the Treasury securities issued to government trust funds, the largest being Social Security. Economists dislike this debt concept, because the trust-fund Treasury securities represent one part of the government owing another. It’s comparable to lending yourself money. Congress could cancel these debts, though it almost certainly won’t. The trust-account Treasury securities represent political commitments more than financial obligations.

(3) Federal loans and loan guarantees: $2.9 trillion in 2011, 19 percent of GDP. The government makes or guarantees loans to college students, farmers, veterans, small businesses and others. The face value of most of these loans don’t show up in the budget, but the government is on the hook if borrowers default. Adding this debt (19 percent of GDP) to gross federal debt produces a total debt ratio of 122 percent of GDP.