Any day now there is going to be a uniquely American political crisis when Congress must raise the limit on how much the federal government may borrow. If it fails to do so, the Treasury department loses the legal authority to issue new bonds.

One peculiarity of the debt limit is that it applies to the gross federal debt, much of which is held by the government itself. That is principally because revenue from the social security tax is greater than is needed to pay current social security benefits. By law, the excess revenue is invested in special Treasury securities.

As of October 19, the gross debt equalled $11,952,611,405,769.02. ($11,953bn). The figure is updated daily on the website of the Treasury's Bureau of the Public Debt. Of this, $4,400bn was held in government accounts and $7,500bn by the public. The public includes the Federal Reserve even though it is a government agency. That is because the Treasury is prohibited by law from selling securities directly to the Fed, which buys them on the open market.